[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.
I changed that to "for some reason" so that I at least don't mislead people. There are at least two bugs involved here, one from TransitionGroup and one from phantom/jasmine/whatever.