pickfirst must move to TransientFailure if it receives empty addresses

[easwars]

Our round_robin LB policy implementation moves to TransientFailure when it receives an update with empty addresses from the resolver (even if it had valid addresses in the past and was Ready). Other languages have the same behavior for round_robin and pick_first.

Our pick_first implementation though, doesn't move to TransientFailure upon receipt of an update with empty addresses if it already has valid addresses. See: https://github.com/grpc/grpc-go/blob/master/pickfirst.go#L54-L59

This needs to change to align with the implementations in other languages.

[easwars]

Fixed by #5274.