Allow null return values for functions to-be-merged.

This is a follow-up to #725 and fixes #883.

If a component and its mixin both define `getInitialState` and one of them returns null, we used to throw (was also covered by a test). This changes it not to throw for any functions that are merged through `createMergedResultFunction`. I'm not entirely sure if this will have unexpected side-effects but our tests seem to comply with the change.