[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.
maybe just do `var mapping = {};` and then

`function(child, key) { if (child != null) mapping[key] = child; }`

if that works?