Targets can be empty if a single excluded IP is provided ("127.0.0.1")

[derpadoo]

• Assign self.targets only if targets_dict["as_nmap"] list contains valid targets. If it's empty, raise a validation error.

https://github.com/rackerlabs/scantron/blob/master/console/django_scantron/models.py#L235

• Ensure coverage in the API check as well.