psql: make %P prompt option consistent when not connected

Hi Hackers,

While reviewing and testing patch [1], I noticed that psql’s %P prompt option does not check pset.db, which makes it inconsistent with other prompt options. As a result, when there is no database connection, other prompt elements are suppressed, but the pipeline status is still shown. For example:

```
evantest=# \set PROMPT1 '[%P] %R '
[off] = \c
connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such file or directory
	Is the server running locally and accepting connections on that socket?
Previous connection kept
[off] = ;
FATAL:  terminating connection due to administrator command
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.
[off] !
[off] !
```

To confirm whether this behavior is intentional, I went through the original discussion for adding %P [2], but didn’t find this case mentioned, so it seems this detail was simply not considered at the time.

This patch adds a pset.db check to align %P with the other prompt options, keeping the prompt display consistent when not connected. With the patch applied, all psql TAP tests pass:
```
# +++ tap check in src/bin/psql +++
t/001_basic.pl ........... ok
t/010_tab_completion.pl .. ok
t/020_cancel.pl .......... ok
t/030_pager.pl ........... ok
All tests successful.
Files=4, Tests=221,  5 wallclock secs ( 0.01 usr  0.01 sys +  1.05 cusr  1.38 csys =  2.45 CPU)
Result: PASS
```
[1] https://www.postgresql.org/message-id/flat/016f6738-f9a9-4e98-bb5a-e1e4b9591d46@uni-muenster.de
[2] https://www.postgresql.org/message-id/CAO6_XqroE7JuMEm1sWz55rp9fAYX2JwmcP_3m_v51vnOFdsLiQ@mail.gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/