pgsql: Fix outfuncs.c to dump A_Const nodes representing NULLs

Log Message:
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Fix outfuncs.c to dump A_Const nodes representing NULLs correctly. This has
been broken since forever, but was not noticed because people seldom look
at raw parse trees. AFAIK, no impact on users except that debug_print_parse
might fail; but patch it all the way back anyway. Per report from Jeff Ross.

Tags:
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REL7_3_STABLE

Modified Files:
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pgsql/src/backend/nodes:
outfuncs.c (r1.176 -> r1.176.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c.diff?r1=1.176&r2=1.176.2.1)