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

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

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/src/bin/pg_dump:
pg_dump.c (r1.400.4.4 -> r1.400.4.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.400.4.4&r2=1.400.4.5)