Re: Fix PQport to never return NULL if the connection is valid

I looked a bit more into the meaning of the port="" setting. The docs
for the port parameter / PGPORT env var
<https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-PORT>
say:

An empty string, or an empty item in a comma-separated list,
specifies the default port number established when
PostgreSQL was built

Thus I understand that the return value of PQport wants to reflect
this behaviour, therefore the value "" is legitimate and it's up to
the client to figure out how the libpq was built (PQconndefaults may
tell that).

Please find attached a new patch that doesn't change the behaviour and
just makes sure to not return NULL in case no info is available in
'conn->connhost'.

Cheers

-- Daniele