pgsql: Fix thinko in my recent change to put an explicit argisrow field

Log Message:
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Fix thinko in my recent change to put an explicit argisrow field in NullTest:
when the planner splits apart a ROW(...) IS NULL test, the argisrow values
of the component tests have to be determined from the component field types,
not copied from the original NullTest (in which argisrow is surely true).

Modified Files:
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    pgsql/src/backend/optimizer/util:
        clauses.c (r1.283 -> r1.284)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.283&r2=1.284)