[nomad-security-group-rules] doesn't allow empty inbound CIDR

[bwalendz]

I noticed that the Nomad SG rules module was lacking the ability to have an empty inbound CIDR. This is useful for when you don't want to explicitly assign CIDRs and have an existing SG you will use and set via var.security_groups

The Vault and Consul equivalents have this ability and it seems rationale be added to the Nomad version as well.

[brikis98]

Agreed! Would you be up for a quick PR to add this?

[bwalendz]

Yes sir.

#60