fix: mime structure with no alternatives

Summary

Use plain mimetext intead of multipart alternatives when no alternative is given

Description

Check if plaintext AND html, only one of them nur neither is given and set the mime-part object accordingly instead of copying the plaintext-part into the html-alternative if omitted as the latter leads to ill-formatted rendering of the html-version.

Checklist

• Have the changes been tested?
  • Behavior if other services fail
• Have tests (e.g. unit tests) been adapted or new ones written? WIP
• Has the changelog been updated?
  • Have the dependencies on other services and their versions been documented?
• Have all changes to configuration variables such as env vars been documented?
• Do all submodules point to stable branches? (develop, master/main)
• Are all pipelines completed successfully?
• Was a review completed by someone else?