The function was tweaked so as it returned one row full of NULLs when
working on an unsupported relkind or an undefined object as of cc53123b,
and after discussion with Amit and Álvaro it looks more natural to make
it return no rows.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Amit Langote
Discussion: https://postgr.es/m/20190227184808.GA17357@alvherre.pgsql