Fix QAction::setFont crash on OSX, when font is unknown.

customMenuFont was null, so objects was an empty array, and
NSDictionary throws an exception when being called with arrays
of different sizes.

Task-number: QTBUG-42728
Change-Id: I8cdab449fd8c1d12b65c46dd5617a7f5e3e96c6e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>