Fix crash when in X11

a281dfdf removed the check on whether
the application was under Wayland or not, unconditionally creating a
connection. Thus, on X11,
KWayland::Client::ConnectionThread::fromApplication would return a null
pointer, crashing the application at m_registry->create(connection).

Thus, check the connection before going further. This prevents
xdg-desktop-portal-kde from crashing under X11.

CCMAIL: aleixpol@kde.org