Don't call ray.init() when num_workers is zero

[ArturNiederfahrenhorst]

Actor Manager calls ray.wait() even when the list of remote_calls is empty and there is nothing to await.
This triggers a ray.init() which is unnecessary. This can be annoying for users who set num_workers=0.

The following script can be run without hitting ray.init() with these changes:

from ray.rllib.algorithms.ppo import PPO, PPOConfig

cfg = PPOConfig().environment("CartPole-v1").framework("torch").rollouts(
    num_rollout_workers=0, validate_workers_after_construction=False)

trainer = PPO(cfg)

trainer.step()