add defaultReleaseType to be None in autorc file

Closes #

Problem with a plugin of auto:

Auto found a commit that looked like a conventional commit, and because the default for unknown labels is to "skip" release, it applied a "skip" mark and computed (correctly) the release as no increment: https://github.com/physiopy/physutils/actions/runs/10497563830/job/29080679203#step:4:611

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