Preserve DOM node when updating empty component

[sophiebits]

Fixes #2770.

Included: Refactor empty component handling; now doesn't use ReactCompositeComponent and ._currentElement is actually null/false.

[sophiebits]

Opinions on the circular dependency here for instantiateReactComponent→ ReactEmptyComponent → instantiateReactComponent? This has instantiateReactComponent pass a reference to itself when instantiating ReactEmptyComponent. (We have this problem in two other places already: in ReactDOMComponent we get around the dependency issue by injecting the cycle via ReactNativeComponent; in ReactCompositeComponent we export an incomplete class and instantiateReactComponent completes it with a reference to itself.)

[sebmarkbage]

Do we need to update anything in devtools?

[sebmarkbage]

WRT the cycles, I think we'll end up merging all cyclic code into ReactReconciler eventually but this is fine for now.

[sophiebits]

Well this is what happened before in the devtools, apparently:

But yes. facebook/react-devtools#199 does that.

[sebmarkbage]

Ok. Lgtm. Let's try it.

Side note: I always thought we allowed true to be empty as well. Isn't that what happens for traverseAllChildren?

[sophiebits]

Yes, that is what happens for real children. We also allow undefined in that case, and empty string makes an empty text node which really could be nothing as well. I was thinking on my way home that maybe we should rescind our support for false and support only null… but maybe supporting everything is simpler, easier, and more consistent.