pgsql: Make stxstattarget nullable

Make stxstattarget nullable

To match attstattarget change (commit 4f622503d6d).  The logic inside
CreateStatistics() is clarified a bit compared to that previous patch,
and so here we also update ATExecSetStatistics() to match.

Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/4da8d211-d54d-44b9-9847-f2a9f1184c76@eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/012460ee93c304fbc7220e5b55d9d0577fc766ab

Modified Files
--------------
doc/src/sgml/catalogs.sgml              | 28 ++++++++++----------
doc/src/sgml/ref/alter_statistics.sgml  | 11 ++++----
src/backend/commands/statscmds.c        | 46 +++++++++++++++++++++-----------
src/backend/commands/tablecmds.c        | 47 +++++++++++++++++----------------
src/backend/parser/gram.y               |  4 +--
src/backend/statistics/extended_stats.c |  4 ++-
src/bin/pg_dump/pg_dump.c               | 10 ++++---
src/bin/psql/describe.c                 |  3 ++-
src/include/catalog/catversion.h        |  2 +-
src/include/catalog/pg_statistic_ext.h  |  6 ++---
src/include/nodes/parsenodes.h          |  2 +-
11 files changed, 93 insertions(+), 70 deletions(-)