Changed null parser to be useful.

Null parser will no longer wrap everything in an UnknownValue instance. If a DataValue instance is
given already, the same instance will be returned. If null is given, null will be returned.
Also extended related tests.

Change-Id: I6dfbf530c9779d3a820d6e0f7b96ef61c9cf7e4f
Merge "Changed null parser to be useful."