AUI-1194 - BaseOptionsCellEditor does not allow empty values

Hey @natecavanaugh,

Attached is an update for http://issues.liferay.com/browse/AUI-1194.

NOTE: This is a backport of https://github.com/liferay/alloy-ui/compare/50efdaf51bc62ca14e418f85030965f78a332cad...e84da6475e404aa07efad95d2dca2ce43d9c9b4c

Please let me know if you have any questions.  Thanks!