[rllib] check if self.env is not None explicitly

[benjamindkilleen]

Why are these changes needed?

The rollout worker stop() method uses if self.env to evaluate whether self.env is None. This can cause errors if the user's env class implements __len__(). I discovered this because I use the same class as a Gym env and as a PyTorch dataset for pretraining, which requires implementing __len__().

Related issue number

Checks

Since I changed only one line, I did not run the unit tests locally, sorry :(

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[benjamindkilleen]

I committed the changes made by the linter, but I don't think I was supposed to. That's why the linter checks are failing, I believe.