Reinstate the (apparently) load-bearing empty bracketed paste (#20037)

Some applications have come to rely on an "empty" bracketed paste packet
to indicate that the clipboard was _requested,_ but not sent. Some
agentic CLI tools use this to guess whether the clipboard contained e.g.
an image or otherwise unserializable data.

Apparently, gnome-terminal and VS Code do this; others as well (I tested
xterm, konsole, gnome-terminal and I can't remember which ones did and
which ones did not.)

Refs #19517