Make \watch respect the user's \pset null setting.

Previously \watch always ignored the user's \pset null setting.
\pset null setting should be ignored for \d and similar queries.
For those, the code can reasonably have an opinion about what
the presentation should be like, since it knows what SQL query
it's issuing. This argument surely doesn't apply to \watch,
so this commit makes \watch use the user's \pset null setting.

Back-patch to 9.3 where \watch was added.

src/bin/psql/command.c

@@ -2868,7 +2868,6 @@ do_watch(PQExpBuffer query_buf, long sleep)
 	 * Set up rendering options, in particular, disable the pager, because
 	 * nobody wants to be prompted while watching the output of 'watch'.
 	 */
-	myopt.nullPrint = NULL;
 	myopt.topt.pager = 0;
 
 	for (;;)