pgsql: Support "" for thousands separator and plus sign in to_char(),

Log Message:
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Support "" for thousands separator and plus sign in to_char(), per
report from French Debian user. psql already handles "" fine.

Modified Files:
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pgsql/src/backend/utils/adt:
formatting.c (r1.104 -> r1.105)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c.diff?r1=1.104&r2=1.105)