[QFontDatabase] Don't look for best foundry twice

In case the foundry name was empty and foundry lookup has failed, don't
look for *any* foundry since it does exactly the same as previous call.

Change-Id: I2a81949df06c31e1ae6999a6bbbe973408d4b814
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>