pgsql: Return NULL for checksum failures if checksums are not enabled

Return NULL for checksum failures if checksums are not enabled

Returning 0 could falsely indicate that there is no problem. NULL
correctly indicates that there is no information about potential
problems.

Also return 0 as numbackends instead of NULL for shared objects (as no
connection can be made to a shared object only).

Author: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Reviewed-by: Robert Treat <rob(at)xzilla(dot)net>

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/252b707bc41cc9bf6c55c18d8cb302a6176b7e48

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 6 ++++--
src/backend/catalog/system_views.sql | 2 +-
src/backend/utils/adt/pgstatfuncs.c | 7 +++++++
src/test/regress/expected/rules.out | 2 +-
4 files changed, 13 insertions(+), 4 deletions(-)