pgsql: Don't bother copying empty support arrays in a zero-column Merge

Don't bother copying empty support arrays in a zero-column MergeJoin.

The case could not arise when this code was originally written, but it can
now (since we made zero-column MergeJoins work for the benefit of FULL JOIN
ON TRUE). I don't think there is any actual bug here, but we might as well
treat it consistently with other uses of COPY_POINTER_FIELD(). Per comment
from Ashutosh Bapat.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d515365a611a58241019c59a62b0cb79584aa725

Modified Files
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src/backend/nodes/copyfuncs.c | 11 +++++++----
1 files changed, 7 insertions(+), 4 deletions(-)