Do not require 'public' to exist for pg_dump -c

Do not require 'public' to exist for pg_dump -c

Commit 330b84d8c4 didn't contemplate the case where the public schema
has been dropped and introduced a query which fails when there is no
public schema into pg_dump (when used with -c).

Adjust the query used by pg_dump to handle the case where the public
schema doesn't exist and add tests to check that such a case no longer
fails.

Back-patch the specific fix to 9.6, as the prior commit was.

Adding tests for this case involved adding support to the pg_dump
TAP tests to work with multiple databases, which, while not a large
change, is a bit much to back-patch, so that's only done in master.

Addresses bug #14650
Discussion: https://www.postgresql.org/message-id/20170512181801.1795.47483%40wrigleys.postgresql.org