fix($parse): return 'undefined' if a middle key's value is null

Prior to this fix, $parse/$eval would return 'null' if a middle key in
an expression's value is null, when it should be expected to be undefined.

This patch tries to remedy this by returning undefined for middle values in
expressions, when fetching a child of that null value.

For example:

```js
// Given the following object:
$scope.a = {
  b: null
};

// $scope.$eval('a.b.c') returns undefined, whereas previously it would return null
```

Closes angular#5480

src/ng/parse.js

@@ -894,16 +894,16 @@ function cspSafeGetterFn(key0, key1, key2, key3, key4, fullExp, options) {
           if (pathVal == null) return pathVal;
           pathVal = pathVal[key0];
 
-          if (!key1 || pathVal == null) return pathVal;
+          if (pathVal == null) return key1 ? undefined : pathVal;
           pathVal = pathVal[key1];
 
-          if (!key2 || pathVal == null) return pathVal;
+          if (pathVal == null) return key2 ? undefined : pathVal;
           pathVal = pathVal[key2];
 
-          if (!key3 || pathVal == null) return pathVal;
+          if (pathVal == null) return key3 ? undefined : pathVal;
           pathVal = pathVal[key3];
 
-          if (!key4 || pathVal == null) return pathVal;
+          if (pathVal == null) return key4 ? undefined : pathVal;
           pathVal = pathVal[key4];
 
           return pathVal;
@@ -924,7 +924,7 @@ function cspSafeGetterFn(key0, key1, key2, key3, key4, fullExp, options) {
             }
             pathVal = pathVal.$$v;
           }
-          if (!key1 || pathVal == null) return pathVal;
+          if (pathVal == null) return key1 ? undefined : pathVal;
 
           pathVal = pathVal[key1];
           if (pathVal && pathVal.then) {
@@ -936,7 +936,7 @@ function cspSafeGetterFn(key0, key1, key2, key3, key4, fullExp, options) {
             }
             pathVal = pathVal.$$v;
           }
-          if (!key2 || pathVal == null) return pathVal;
+          if (pathVal == null) return key2 ? undefined : pathVal;
 
           pathVal = pathVal[key2];
           if (pathVal && pathVal.then) {
@@ -948,7 +948,7 @@ function cspSafeGetterFn(key0, key1, key2, key3, key4, fullExp, options) {
             }
             pathVal = pathVal.$$v;
           }
-          if (!key3 || pathVal == null) return pathVal;
+          if (pathVal == null) return key3 ? undefined : pathVal;
 
           pathVal = pathVal[key3];
           if (pathVal && pathVal.then) {
@@ -960,7 +960,7 @@ function cspSafeGetterFn(key0, key1, key2, key3, key4, fullExp, options) {
             }
             pathVal = pathVal.$$v;
           }
-          if (!key4 || pathVal == null) return pathVal;
+          if (pathVal == null) return key4 ? undefined : pathVal;
 
           pathVal = pathVal[key4];
           if (pathVal && pathVal.then) {
@@ -980,7 +980,7 @@ function simpleGetterFn1(key0, fullExp) {
   ensureSafeMemberName(key0, fullExp);
 
   return function simpleGetterFn1(scope, locals) {
-    if (scope == null) return scope;
+    if (scope == null) return undefined;
     return ((locals && locals.hasOwnProperty(key0)) ? locals : scope)[key0];
   };
 }
@@ -990,10 +990,9 @@ function simpleGetterFn2(key0, key1, fullExp) {
   ensureSafeMemberName(key1, fullExp);
 
   return function simpleGetterFn2(scope, locals) {
-    if (scope == null) return scope;
+    if (scope == null) return undefined;
     scope = ((locals && locals.hasOwnProperty(key0)) ? locals : scope)[key0];
-
-    return scope == null ? scope : scope[key1];
+    return scope == null ? undefined : scope[key1];
   };
 }
 
@@ -1036,7 +1035,7 @@ function getterFn(path, options, fullExp) {
     var code = 'var p;\n';
     forEach(pathKeys, function(key, index) {
       ensureSafeMemberName(key, fullExp);
-      code += 'if(s == null) return s;\n' +
+      code += 'if(s == null) return undefined;\n' +
               's='+ (index
                       // we simply dereference 's' on any .dot notation
                       ? 's'

test/ng/parseSpec.js

@@ -977,6 +977,55 @@ describe('parser', function() {
             expect($parse('"name" + id').constant).toBe(false);
           }));
         });
+
+        describe('nulls in expressions', function() {
+          // simpleGetterFn1
+          it('should return null for `a` where `a` is null', inject(function($rootScope) {
+            $rootScope.a = null;
+            expect($rootScope.$eval('a')).toBe(null);
+          }));
+
+          it('should return undefined for `a` where `a` is undefined', inject(function($rootScope) {
+            expect($rootScope.$eval('a')).toBeUndefined();
+          }));
+
+          // simpleGetterFn2
+          it('should return undefined for properties of `null` constant', inject(function($rootScope) {
+            expect($rootScope.$eval('null.a')).toBeUndefined();
+          }));
+
+          it('should return undefined for properties of `null` values', inject(function($rootScope) {
+            $rootScope.a = null;
+            expect($rootScope.$eval('a.b')).toBeUndefined();
+          }));
+
+          it('should return null for `a.b` where `b` is null', inject(function($rootScope) {
+            $rootScope.a = { b: null };
+            expect($rootScope.$eval('a.b')).toBe(null);
+          }));
+
+          // cspSafeGetter && pathKeys.length < 6 || pathKeys.length > 2
+          it('should return null for `a.b.c.d.e` where `e` is null', inject(function($rootScope) {
+            $rootScope.a = { b: { c: { d: { e: null } } } };
+            expect($rootScope.$eval('a.b.c.d.e')).toBe(null);
+          }));
+
+          it('should return undefined for `a.b.c.d.e` where `d` is null', inject(function($rootScope) {
+            $rootScope.a = { b: { c: { d: null } } };
+            expect($rootScope.$eval('a.b.c.d.e')).toBeUndefined();
+          }));
+
+          // cspSafeGetter || pathKeys.length > 6
+          it('should return null for `a.b.c.d.e.f.g` where `g` is null', inject(function($rootScope) {
+            $rootScope.a = { b: { c: { d: { e: { f: { g: null } } } } } };
+            expect($rootScope.$eval('a.b.c.d.e.f.g')).toBe(null);
+          }));
+
+          it('should return undefined for `a.b.c.d.e.f.g` where `f` is null', inject(function($rootScope) {
+            $rootScope.a = { b: { c: { d: { e: { f: null } } } } };
+            expect($rootScope.$eval('a.b.c.d.e.f.g')).toBeUndefined();
+          }));
+        });
       });
     });
   });