pgsql: Repair bug #2694 concerning an ARRAY[] construct whose inputs are

Log Message:
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Repair bug #2694 concerning an ARRAY[] construct whose inputs are empty
sub-arrays. Per discussion, if all inputs are empty arrays then result
must be an empty array too, whereas a mix of empty and nonempty arrays
should (and already did) draw an error. In the back branches, the
construct was strict: any NULL input immediately yielded a NULL output;
so I left that behavior alone. HEAD was simply ignoring NULL sub-arrays,
which doesn't seem very sensible. For lack of a better idea it now
treats NULL sub-arrays the same as empty ones.

Modified Files:
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pgsql/src/backend/executor:
execQual.c (r1.196 -> r1.197)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.196&r2=1.197)