pgsql: transformColumnDefinition failed to complain about create table

Log Message:
-----------
transformColumnDefinition failed to complain about
	create table foo (bar int default null default 3);
due to not thinking about the special-case handling of DEFAULT NULL.
Problem noticed while investigating bug #3396.

Tags:
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REL8_2_STABLE

Modified Files:
--------------
    pgsql/src/backend/parser:
        analyze.c (r1.353 -> r1.353.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c.diff?r1=1.353&r2=1.353.2.1)