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

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.

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