wayland: Allow a NULL inhibitors hash table

wayland: Allow a NULL inhibitors hash table

The shortcuts inhibitors hash table is created when we create a
GdkWaylandWindow implementation for a GdkWindow, and it's destroyed once
we finalize the instance. The fake "root" window we create for the
Wayland display does not have a backing native window, so the shortcuts
inhibitors hash table is set to NULL; this causes a critical error
message when calling g_hash_table_destroy() on it. The finalization of
the root window happens when we close a display connection.

We should use g_clear_pointer(), instead, as it's NULL safe.

Without this change, the displayclose test fails, as all warnings are
considered fatal.