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