VACUUM VERBOSE: tweak scanned_pages logic.

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.