cqltypes: Serialize None values in collections as NULLs

Fixes scylladb#201

When using parepared statements, None values in collections
were serialized as empty values (values with length == 0). This is unexpected and
inconsistent - None values are serialized as NULLs (vlaues with length == -1) in other cases:
 - Statement arguments, both for simple and prepared statements
 - Collection elements in simple statement

This commit fixes this weird behavior - now None values should be serialized as NULLs in all cases.

cassandra/cqltypes.py

@@ -832,9 +832,12 @@ def serialize_safe(cls, items, protocol_version):
         buf.write(pack(len(items)))
         inner_proto = max(3, protocol_version)
         for item in items:
-            itembytes = subtype.to_binary(item, inner_proto)
-            buf.write(pack(len(itembytes)))
-            buf.write(itembytes)
+            if item is None:
+                buf.write(int32_pack(-1))
+            else:
+                itembytes = subtype.to_binary(item, inner_proto)
+                buf.write(pack(len(itembytes)))
+                buf.write(itembytes)
         return buf.getvalue()
 
 
@@ -904,10 +907,16 @@ def serialize_safe(cls, themap, protocol_version):
         for key, val in items:
             keybytes = key_type.to_binary(key, inner_proto)
             valbytes = value_type.to_binary(val, inner_proto)
-            buf.write(pack(len(keybytes)))
-            buf.write(keybytes)
-            buf.write(pack(len(valbytes)))
-            buf.write(valbytes)
+            if keybytes is not None:
+                buf.write(pack(len(keybytes)))
+                buf.write(keybytes)
+            else:
+                buf.write(int32_pack(-1))
+            if valbytes is not None:
+                buf.write(pack(len(valbytes)))
+                buf.write(valbytes)
+            else:
+                buf.write(int32_pack(-1))
         return buf.getvalue()