[pgAdmin][RM4642] port should not be mandatory when a service is provided

Hi Hackers,

Attached is the patch to fix the RM #4642 "port should not be mandatory
when a service is provided". *Cyril Jouve* has sent the initial patch, but
that only removes the validation on GUI, when we save the server properties
without port and username backend throws an error because port and username
is *NOT NULL* columns in the server table in SQLite.

I have removed the *NOT NULL *constraint from the port and username. The
maintenance database is required as our whole connection logic is based on
that, so I have added the NOT NULL constraint for that column.

Please review it.

--
*Thanks & Regards*
*Akshay Joshi*

*Sr. Software Architect*
*EnterpriseDB Software India Private Limited*
*Mobile: +91 976-788-8246*