Fix `findComponentRoot` w/ Unidentified Nodes

The current `ReactInstanceHandles` has a bug where `findComponentRoot` barfs if it comes across a node that was not identified by React (via `ReactID`). This fixes that.

This was always a bug, but it became more apparent once we switched to `data-reactid` because arbitrary `document.createElement`'d nodes are much more likely to have an `id` than they are to have a `data-reactid`.