Tighten usage of PSQL_WATCH_PAGER.

Tighten usage of PSQL_WATCH_PAGER.

Don't use PSQL_WATCH_PAGER when stdin/stdout are not a terminal.
This corresponds to the restrictions on when other commands will
use [PSQL_]PAGER.  There isn't a lot of sense in trying to use a
pager in non-interactive cases, and doing so allows an environment
setting to break our tests.

Also, ignore PSQL_WATCH_PAGER if it is set but empty or all-blank,
for the same reasons we ignore such settings of [PSQL_]PAGER (see
commit 18f8f784c).

No documentation change is really needed, since there is nothing
suggesting that these constraints on [PSQL_]PAGER didn't already
apply to PSQL_WATCH_PAGER too.  But I rearranged the text
a little to make it read more naturally (IMHO anyway).

Per report from Pavel Stehule.  Back-patch to v15 where
PSQL_WATCH_PAGER was introduced.

Discussion: https://postgr.es/m/CAFj8pRDTwFzmEWdA-gdAcUh0ZnxUioSfTMre71WyB_wNJy-8gw@mail.gmail.com