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.