Fix table dump in pg_dump[all] with backends older than 9.5

Fix table dump in pg_dump[all] with backends older than 9.5

The access method name "amname" can be dumped as of 3b925e90, but
queries for backends older than 9.5 forgot to map it to a dummy NULL
value, causing the column to not be mapped to a number.  As a result,
pg_dump was throwing some spurious errors in its stderr output coming
from libpq:
pg_dump: column number -1 is out of range 0..36

Fix this issue by adding a mapping of "amname" to NULL to all the older
queries.

Discussion: https://postgr.es/m/20190522083038.GA16837@paquier.xyz
Author: Michael Paquier
Reviewed-by: Dmitry Dolgov, Andres Freund, Tom Lane