fix: Add a default value for dependabot_filename_to_use

[gbrindisi]

Pull Request

Fix https://github.com/github/evergreen/issues/168

Proposed Changes

Set a default value to dependabot_filename_to_use rather than None

Readiness Checklist

Author/Contributor

• [x] If documentation is needed for this change, has that been included in this pull request
• [x] run make lint and fix any issues that you have introduced
• [x] run make test and ensure you have test coverage for the lines you are introducing
• [ ] If publishing new data to the public (scorecards, security scan results, code quality results, live dashboards, etc.), please request review from @jeffrey-luszcz

Reviewer

• [x] Label as either fix, documentation, enhancement, infrastructure, maintenance or breaking