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

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