fix: return the compiled port if PQport returns an empty string

fix: return the compiled port if PQport returns an empty string

Looking at the docs this seems the default value when an empty string is
specified, as opposite as the value in the PGPORT env var. See
<https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-PORT>.

This change is an improvement on top of 40dfa0b, where we allowed PGport
to return NULL. This is likely a libpq bug (reported in
<https://www.postgresql.org/message-id/CA%2Bmi_8YTS8WPZPO0PAb2aaGLwHuQ0DEQRF0ZMnvWss4y9FwDYQ%40mail.gmail.com>)
but now it's in the wild so we better support it rather than dying of
assert or by int()'ing an empty string.