Avoid sending data offers from an invalid source.
Summary: A DataDevice will have a source when offers are available, but it can also be legitimately cleared. When calling DataDeviceInterface::sendSelection(DataDeviceInterface *other) if the other data device has no source, we should be setting that we also have no source. In addition this also guards against Seat tracking a DataDeviceInterface with no source when trying to sync x clipboards. BUG: 383054 Reviewers: #plasma Subscribers: graesslin, plasma-devel, #frameworks Tags: #plasma_on_wayland, #frameworks Differential Revision: https://phabricator.kde.org/D7316