recipe/postgres: fix regression with empty port

32fbf0d4 (recipe/postgres: support non standard port, 2021-11-12)
broke support for empty port, it was generating config file with
`port = ` , which is an invalid config file syntax for postgresql.
7 jobs for fix/postgres-empty-port in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:56:36

passed SlapOS.Eggs.UnitTest-Jerome.Python3

01:30:21

running SlapOS.Eggs.UnitTest-Master.Python2

26218:16:47

running SlapOS.Eggs.UnitTest-Master.Python3

26218:15:59

running SlapOS.SlapRunner.ResilienceTest-Master

26218:16:36

running SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

26218:16:45

running SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

26218:16:35