vscclient: do not add a socket watch if there is not data to send
vscclient: do not add a socket watch if there is not data to send

Fixes the following error:
** (process:780): CRITICAL **: do_socket_send: assertion
`socket_to_send->len != 0' failed

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>
Signed-off-by: Gerd Hoffmann <cad73e70082427fddc9be3baaf34e698dffd21c0@redhat.com>

SPICE/libcacard,elmarco/libcacard,elmarco/libcacard,SPICE/libcacard