pgsql: Reimplement nullification of walsender timestamp

Reimplement nullification of walsender timestamp

Make the value null only at pg_stat_activity-output time, as suggested
by Tom Lane, instead of messing with the internal state. This should
appease buildfarm members with force_parallel_mode=regress, which are
running parallel queries on logical replication walsenders.

The fact that walsenders can run parallel queries should perhaps be
studied more carefully, but for the moment let's get rid of the red
blots in buildfarm.

Backpatch to pg10, like the previous commit.

Discussion: https://postgr.es/m/[email protected]

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/20c4df8c8d7c54e34e921e770b594582d276f19e

Modified Files
--------------
src/backend/access/transam/xact.c | 7 -------
src/backend/utils/adt/pgstatfuncs.c | 8 +++++++-
2 files changed, 7 insertions(+), 8 deletions(-)