Return the libpq default port when blank in conninfo

PQport may return null or empty string when the same is set in
connection options. Internally, libpq falls back to its configured
default any time it reads these.

ext/pg_connection.c

@@ -702,7 +702,10 @@ static VALUE
 pgconn_port(VALUE self)
 {
 	char* port = PQport(pg_get_pgconn(self));
-	return INT2NUM(atoi(port));
+	if (!port || port[0] == '\0')
+		return INT2NUM(DEF_PGPORT);
+	else
+		return INT2NUM(atoi(port));
 }
 
 /*