[src/server] Verify that surface has a resource before sending pointer enter

Summary:
If we try to send a pointer enter on a null resource of a Surface
(e.g. after unbound) we hit a marshalling error:
error marshalling arguments for enter (signature uoff): null value passed for arg 1

The added test case hits this error without the change and passes
cleanly with the change.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2159