[server] Add resource check in QtExtendedSurfaceInterface::close

Summary:
The QtExtendedSurfaceInterface might be unbound and thus the resource
might be null when calling into close. Thus we need to do a nullptr
check.

Hit a crash there.

Test Plan:
Unfortunately no test case as we don't have a client side
implementation for this.

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D1961