Prevent bitmap heap scans from showing unnecessary block info in EXPLAIN ANALYZE.

EXPLAIN ANALYZE shows the information of the numbers of exact/lossy blocks which
bitmap heap scan processes. But, previously, when those numbers were both zero,
it displayed only the prefix "Heap Blocks:" in TEXT output format. This is strange
and would confuse the users. So this commit suppresses such unnecessary information.

Backpatch to 9.4 where EXPLAIN ANALYZE was changed so that such information was
displayed.

Etsuro Fujita