Do not return NULL from pg_newlocale_from_collation().

Do not return NULL from pg_newlocale_from_collation().

Previously, pg_newlocale_from_collation() returned NULL as a special
case for the DEFAULT_COLLATION_OID if the provider was libc. In that
case the behavior would depend on the last call to setlocale().

Now, consistent with the other providers, it will return a pointer to
default_locale, which is not dependent on setlocale().

Note: for the C and POSIX locales, the locale_t structure within the
pg_locale_t will still be zero, because those locales are implemented
with internal logic and do not use libc at all.

lc_collate_is_c() and lc_ctype_is_c() still depend on setlocale() to
determine the current locale, which will be removed in a subsequent
commit.

Discussion: https://postgr.es/m/cfd9eb85-c52a-4ec9-a90e-a5e4de56e57d@eisentraut.org
Reviewed-by: Peter Eisentraut, Andreas Karlsson