VACUUM VERBOSE: tweak scanned_pages logic.

Commit 872770fd6c taught VACUUM VERBOSE and autovacuum logging to
display the total number of pages scanned by VACUUM.  This information
was also displayed as a percentage of rel_pages in parenthesis, which
makes it easy to spot trends over time and across tables.

The instrumentation displayed "0 scanned (0.00% of total)" for totally
empty tables.  Tweak the instrumentation: have it show "0 scanned
(100.00% of total)" for empty tables instead.  This approach is clearer
and more consistent.

src/backend/access/heap/vacuumlazy.c

@@ -676,7 +676,7 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 							 vacrel->removed_pages,
 							 vacrel->rel_pages,
 							 vacrel->scanned_pages,
-							 orig_rel_pages == 0 ? 0 :
+							 orig_rel_pages == 0 ? 100.0 :
 							 100.0 * vacrel->scanned_pages / orig_rel_pages);
 			appendStringInfo(&buf,
 							 _("tuples: %lld removed, %lld remain, %lld are dead but not yet removable\n"),