Summary:
When starting with an empty table and adding rows, the scrollOffset of the table might end up with a negative value, which causes rows not to be rendered.
See https://github.com/bvaughn/react-window/issues/49 for more details.
This only calls `scrollToItem` when the table is actually scrollable to prevent a negative scrollOffset.
Reviewed By: passy
Differential Revision: D9570518
fbshipit-source-id: 5b08990a353a6d41d0f1bc5c1481d5a5e471bd94