GdkWin32: Use primary monitor as fallback if popup layout does not intersect any
Similar to what's done in the macOS backend: https://gitlab.gnome.org/GNOME/gtk/-/blob/4.7.2/gdk/macos/gdkmacospopupsurface.c#L67-68.
Fixes a crash caused by attempting to dereference a NULL GdkMonitor*. See #5125