BreakoutWidget sometimes breaks when clicking date/time field without picking a granularity

[tlrobinson]

I'm not exactly sure what conditions trigger this, but on certain tables if you click a date/time field in the breakout widget you can get error message like this:

invariant.js?568c:39 Uncaught Invariant Violation: BreakoutWidget.render(): A valid ReactComponent must be returned. You may have returned undefined, an array or some other invalid object.

However, if you click the right side so you drill into the date/time granularity picker and pick "day", "week", etc it works fine.

This appears to be due to the lack of an "else" statement here: https://github.com/metabase/metabase/blob/master/frontend/src/query_builder/BreakoutWidget.jsx#L94