ci: update conditions on when to auto assign reviewer

[daniel-weisse]

Context

Lots of dependency PRs don't get assigned a reviewer. I hope this PR fixes some of the cases where the review assignment is skipped.

Proposed change(s)

• Add more triggers to the assign_reviewer workflow
• Only run the workflow if no reviewers are assigned
• Update the name we check against to renovate[bot] following this comment: https://github.com/renovatebot/renovate/discussions/13704#discussioncomment-7079564

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[daniel-weisse]

Managed to fix the workflow:

• For some reason github.event.pull_request.user.login is renovate[bot] not app/renovate like the gh CLI would suggest
• github.event.pull_request.requested_reviewers by itself simply evaluates to Array. We need to call toJson on it to parse it to a JSON object, at which point we can evaluate if that structure is an empty array. But not actually, because its JavaScript we check if it is a string equal to '[]'