Relaxed the functional If op to allow no outputs.

One use case is that in Swift for TensorFlow, we generate an If op that's
executed only for its side effects.

The Swift level code looks like:

if some_cond {
  sendTensorToHost(1.0)
} else {
  sendTensorToHost(2.0)
}

Currently we hit the following error in TF, because a attr list by default
requires a length of >=1.

PiperOrigin-RevId: 203063925