pgsql: Use just NULL not NULL::TEXT --- the latter coding is unnecessary

Log Message:
-----------
Use just NULL not NULL::TEXT --- the latter coding is unnecessary and
not schema-safe. Per report from Jochem van Dieten.

Modified Files:
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pgsql/src/bin/pg_dump:
pg_dump.c (r1.408 -> r1.409)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.408&r2=1.409)