[pgAdmin4][RM#3200] Don't set empty string for host/hostaddr in model when service is used

[Murtuza Zabuawala]

Hi,

PFA patch to fix the issue where pgAdmin4 was not able to utilise the host
parameter from service file.

--
Regards,
Murtuza Zabuawala
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

RM_3200.diff
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