Rework code using list_delete_cell() in MergeAttributes

Rework code using list_delete_cell() in MergeAttributes

When merging two attributes, we are sure that at least one remains.
However, when deleting one element in the attribute list we may finish
with an empty list returned as NIL by list_delete_cell(), but the code
failed to track that, which is not project-like.  Adjust the call so as
we check for an empty list, and make use of it in an assertion.

This has been introduced by e7b3349, when adding support for CREATE
TABLE OF.

Author: Mark Dilger
Reviewed-by: Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/CAE-h2TpPDqSWgOvfvSziOaMngMPwW+QZcmPpY8hQ_KOJ2+3hXQ@mail.gmail.com