[xdgoutput] Only send initial name and description if set

Summary:
We hit this if we run an old kwin against new kwayland.

Kwin won't set the name so it's empty, so we'll call
zxdg_output_v1_send_name with an empty string

QtWayland will see that we receive this and update mOutputName to an
empty string which causes client issues.

Reviewers: KWin

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28245