Fix issues with empty sources
* Validate that each permission on traffic permissions resources has at least one source.
* Don't construct RBAC policies when there aren't any principals. This resulted in Envoy rejecting xDS updates with a validation error.
```
error=
| rpc error: code = Internal desc = Error adding/updating listener(s) public_listener: Proto constraint validation failed (RBACValidationError.Rules: embedded message failed validation | caused by RBACValidationError.Policies[consul-intentions-layer4-1]: embedded message failed validation | caused by PolicyValidationError.Principals: value must contain at least 1 item(s)): rules {
```
Eric Haberkorn