[Addons] Fix TransitionGroup null children bug

Fixes #1949

When we render two children to two nulls, `ReactTransitionChildMapping.getChildMapping` now returns `{}` rather than `{'.0': null, '.1': null}`. Otherwise mergeChildMapping doesn't doesn't understand that the children have disappeared.
Nope, the key being passed would be the index, not the final key assigned to the child. @sebmarkbage