Improve a few things in pg_walinspect

Improve a few things in pg_walinspect

This improves a few things in pg_walinspect:
- Return NULL rather than empty strings in pg_get_wal_records_info() for
the block references and the record description if there is no
information provided by the fallback.  This point has been raised by
Peter Geoghegan.
- Add a check on XLogRecHasAnyBlockRefs() for pg_get_wal_block_info(),
to directly skip records that have no block references.  This speeds up
the function a bit, depending on the number of records that have no
block references.

Author: Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACWL9RG8sGJHinggRNBTxgRWJTSxCkB+cE6=t3Phh=Ey+A@mail.gmail.com