Summary:
Even if the XDG_RUNTIME_DIR is not set, KWayland should not crash
if methods are invoked which only make sense after the server was
started successfully. This was not yet the case for
Display::dispatchEvents.

Also the wl_display was not destroyed in the situation that creating
the socket failed.

Test Plan:
Test case added which simulates the situation of no
XDG_RUNTIME_DIR being set.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2077