Fix handling of the case where specified surface can't imported

An xdg_imported resource must be created no matter whether there is an
exported surface with the specified handle.

This change fixes handling of that case in kwaylandserver by introducing
a dummy imported object.

If there is no exported surface with the given handle, the compositor
should create an xdg_imported resource and immediately send the
destroyed event.

A dummy imported object is needed because generated code sets resource
implementation; otherwise we would set an implementation without any
user data to mark the resource as inert.