shallowEqual: bail if either argument is falsey

[ianobermiller]

Also add some unit tests.

Fixes #3316

[jimfb]

I like this PR, especially since it is predominantly adding unit tests. I'm going to mark it as accepted, but pinging @spicyj just in case I missed something (the semantics of javascript falsey expressions are subtle). We are in a code-freeze for the 0.13 release, so ETA for a merge is a week-ish.

[syranide]

This seems weird to me I think, why do we want this behavior?

[sophiebits]

@syranide Why not?

[jimfb]

@syranide returning false (if one object is null and the other is not) seems more correct than throwing an unexpected exception.

[syranide]

@JSFB Ok, after reading it a bit more I'm kind of OK with it, but it's not super obvious, I'd say it could make more sense to make it objA == null || objB == null for clarity? shallowEqual(1, 2) === true. props and state can indeed be non-objects too, although perhaps that is being restricted.