format: allow tap to not have ipv4 address

fixup for a8917a3c
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS-Master.UnitTest-Master

01:15:47

failed SlapOS.Eggs.UnitTest-Master.Python2

00:11:57

failed SlapOS.Eggs.UnitTest-Master.Python3

00:12:55

failed SlapOS.Eggs.UnitTest-Master.Python2

00:14:45

failed SlapOS.Eggs.UnitTest-Master.Python3

00:13:22