macosx: do not warn when trying to translate null strings

We don't do any actual call to gettext here, so the warning
is not really needed.

modules/gui/macosx/StringUtility.m

@@ -65,7 +65,6 @@ static VLCStringUtility *_o_sharedInstance = nil;
             return(@"");
         }
     } else {
-        msg_Warn(VLCIntf, "can't translate empty strings");
         return(@"");
     }