shell-window-tracker: Detect invalid PID values of 0

MetaWindows get_pid() API changed to use the client PID, which also
works for Wayland clients instead of only X11 clients now. This API
returns 0 instead of -1 for invalid PIDs, so update our check according
to that.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1184

src/shell-window-tracker.c

@@ -343,7 +343,7 @@ get_app_from_window_pid (ShellWindowTracker  *tracker,
 
   pid = meta_window_get_pid (window);
 
-  if (pid == -1)
+  if (pid < 1)
     return NULL;
 
   result = shell_window_tracker_get_app_from_pid (tracker, pid);