pgsql: Fix initialization of pg_stat_get_lastscan()

Fix initialization of pg_stat_get_lastscan()

A NULL result should be reported when a stats timestamp is set to 0, but
c037471 missed that, leading to a confusing timestamp value after for
example a DML on a freshly-created relation with no scans done on it
yet.

This impacted the following attributes for two system views:
- pg_stat_all_tables.last_idx_scan
- pg_stat_all_tables.last_seq_scan
- pg_stat_all_indexes.last_idx_scan

Reported-by: Robert Treat
Analyzed-by: Peter Eisentraut
Author: Dave Page
Discussion: https://postgr.es/m/CABV9wwPzMfSaz3EfKXXDxKmMprbxwF5r6WPuxqA=5mzRUqfTGg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d7744d50a5394db8d973d1cfa034ed6640aec13c

Modified Files
--------------
src/backend/utils/adt/pgstatfuncs.c | 8 +++++++-
src/test/regress/expected/stats.out | 6 ++++++
src/test/regress/sql/stats.sql | 1 +
3 files changed, 14 insertions(+), 1 deletion(-)