grpclb: turn into TRANSIENT_FAILURE if given (by balancer or fallback) an empty list of addresses

[voidzcy]

Fixes #7935

There are two cases gRPCLB needs to handle an empty list of backend addresses:

• The remote balancer gives an empty list.

  • grpclb: handles empty address from LB #6734 partially addressed the issue, but it only solved pick_first's immediate problem. Also, that change makes round_robin and pick_first diverge in terms of the behavior for receiving an empty list of backend addresses: round_robin buffers RPCs (which could potentially hang forever as fallback timer had already been cancelled when receiving the response) while pick_first fails RPCs immediately.

• The fallback list given by the resolver, when it is used, turns out to be empty. This is what gRPC-LB without fallback addresses can hang #7935 is describing.

  • In this case, both round_robin and pick_first buffer RPCs.

This PR fixes the entire problem. It turns into TRANSIENT_FAILURE as long as the list of backends to be used is found to be empty.

There could be other choices of behaviors such as do not cancel the fallback timer if balancer gives an empty list and only turn into TF if fallback also fail. The choice in this PR is the simplest handling, we don't unnecessarily complicate ourselves.

[ejona86]

There's still the style stuff I suggested before, but LGTM