[server] Ensure that Resource::Private::get returns nullptr if passed a nullptr

Summary:
A Resource might be unbound, but not yet destroyed. In that case this
return a Resource instead of nullptr.

This change adds an explicit nullptr check.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D1938