fix: check for empty diff (#1273)

* Fix check for empty diff

Compare to the result of the latest diff module: https://runkit.com/embed/y4qzdzzz05qr

No diff gives this result:

```
Index: 
===================================================================
--- 	
+++ 	
```

* Actually check the right thing in the fix as well

src/commands/readme.js

@@ -101,7 +101,7 @@ module.exports.handler = function readme(argv) {
         '',
         ''
       );
-      if (!diffRaw.length) {
+      if (diffRaw.split('\n').length === 5) {
         log(`${argv.readmeFile} is up to date.`);
         process.exit(0);
       }