It's unnecessary to return an actual slot when we have no tuple.
We can just return NULL, which avoids the risk of indexing into an
array that might not contain any elements.

Rushabh Lathia, per a report from Tomas Vondra

Discussion: http://postgr.es/m/6ecd6f17-0dcf-1de7-ded8-0de7db1ddc88@2ndquadrant.com