pgsql: Enable pg_collation_actual_version() to work on the default coll

Enable pg_collation_actual_version() to work on the default collation.

Previously, it would simply return NULL, which was less useful.

Discussion: https://postgr.es/m/51fb77507cafd43fc1a2e733c23045873d93ae60.camel%40j-davis.com
Reviewed-by: Thomas Munro

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/10932ed5e57b75e5e9aaa22aa0527ceeee55b6ca

Modified Files
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src/backend/commands/collationcmds.c | 67 ++++++++++++++++++++++++++----------
1 file changed, 48 insertions(+), 19 deletions(-)