Fix crash on pasting too soon after copying from XWayland

There seems to be an interval between when you copy something from a Wayland
client and when you attempt to paste it into an X client where m_chunks.first().first
is empty, and trying to access its .data() will cause an assertion. While we can't
really gracefully handle this situation, we can at least terminate the function early
and turn the paste operation into a noop instead of a crash.

(cherry picked from commit c6b63826)