Fix stop parameter check to allow explicit None value

[sam-baumann]

Previously, not self.generator_kwargs.get("stop") would overwrite a user-supplied stop=None (which disables the stop sequence). Use "stop" not in to only set the default when the key is absent.

Thinking models, at least gemma4:e2b in Ollama, will stop their reasoning chain on \n even if they shouldn't complete, so this gives a way to overwrite that

What does this PR do?

checks if "stop" is a member of the generator_kwargs dict instead of just checking it as a boolean, as passing in None caused it to evaluate to False and it sets stop as \n anyway

Fixes #2481

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case. Thinking models in ollama exit early #2481 submitted alongside this PR
• Did you make sure to update the documentation with your changes (if applicable)? N/A
• Did you write any new necessary tests? N/A