Optional resource_id for ai actions

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/414616

Deal with optional resource_id in service and worker

Makes sure we allow using nil as resource_id in the service and the worker.

Screenshots or screen recordings

N/A

Before	After

How to set up and validate locally

1. Checkout the branch
2. Validate that chat works as expected

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.