xwayland: Match applications without WM_CLASS nor WM_NAME
For X11 grabs, the pattern matching mechanism would simply ignore applications which have neither WM_CLASS nor WM_NAME set.
When dealing with an override redirect window however, it is not uncommon that these window have neither value set as these window are supposed to be ignored by the window manager.
When the WM_CLASS or the WM_NAME is not set by the client, assume the value is empty so the pattern matching can allow for these.