pgsql: Allow NULL version for individual collations.

Allow NULL version for individual collations.

Remove the documented restriction that collation providers must either
return NULL for all collations or non-NULL for all collations.

Use NULL for glibc collations like "C.UTF-8", which might otherwise lead
future proposed commits to force unnecessary index rebuilds.

Reviewed-by: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>
Discussion: https://postgr.es/m/CA%2BhUKGJvqup3s%2BJowVTcacZADO6dOhfdBmvOPHLS3KXUJu41Jw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/382a821907e76bf6068492a164fdfc57578391f4

Modified Files
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src/backend/utils/adt/pg_locale.c | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)