fix: error when updating ruleset without bypass actors

[dblinkhorn]

Resolves #2803

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Before the change?

• Updating a github_repository_ruleset that never had bypass_actors defined would fail with a 422 Invalid request. Invalid property /bypass_actors: data cannot be null. error
• This bug was introduced in v6.7.0 when fixing the original bypass actors deletion bug in fix: bypass actors not being deleted from repository rulesets #2780
• The provider was always using UpdateRulesetNoBypassActor, which sends bypass_actors: null when the field was never defined, causing GitHub API to reject the request

After the change?

• Rulesets without bypass_actors can now be updated without errors
• The provider conditionally uses UpdateRulesetNoBypassActor only when d.HasChange("bypass_actors") is true
• When bypass_actors hasn't changed, uses UpdateRuleset which omits the field entirely
• Adds a new test case to ensure bug is not re-introduced

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[nickfloyd]

❤️