pgsql: Fix ANALYZE to accumulate some minimal statistics for an all-null

Log Message:
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Fix ANALYZE to accumulate some minimal statistics for an all-null column.
Per gripes from Mike Mascari and Bernd Heller.

Modified Files:
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pgsql/src/backend/commands:
analyze.c (r1.81 -> r1.82)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c.diff?r1=1.81&r2=1.82)