Return True when there are no user backends during shutdown - #3724

When PostgreSQL has no user backends, wait_for_user_backends_to_close() implicitly returns None, so _do_stop() skips the early on_safepoint callback. The watchdog can remain armed while PostgreSQL completes its remaining shutdown work.

Return True explicitly for the empty-backend case, as documented. Keep the existing behavior for active backends, timeouts, and failures to retrieve child processes unchanged.

Add regression coverage for empty child lists and auxiliary-only/disappeared children, with and without a timeout. Also verify that the safepoint callback runs before waiting for the postmaster to exit, and clarify this behavior in the watchdog documentation.

Fixes #3723.

Validation on Windows / Python 3.12:

• Full unit suite and doctests (pytest tests --doctest-modules patroni): 754 passed, 17 skipped, 39 subtests passed.
• Full flake8 check (flake8 patroni tests setup.py): passed.
• git diff --check: passed.
• Before the fix, the new regression coverage failed in all six empty-backend subcases.

Linux watchdog and PostgreSQL/DCS integration scenarios have not been run locally.