Don't fail on tag filtering when group is not specified.

Fixing the problem introduced in change "JCLOUDS-367: GCE nodes n>1 ignoring inboundPort".
This test setup still seems _very_ complicated, but if that's really the easiest way to test it, then so be it ;-) Just one question: from the code, it looks like we have three test cases:

1. Group null - no filtering applied
2. Group non-null and `firewallTagNamingConvention.get(group)` _is_ a firewall tag - needs to be filtered
3. Group non-null and `firewallTagNamingConvention.get(group)` is _not_ a firewall tag - shoudl not be filtered

I can see that cases 1. and 2. ("Group-port-42" is removed) are covered. Does "aTag" cover case 3., or are we missing that?