allow empty body when attachment is provided

[gera2ld]

Description:

Related issue (if applicable): refs #

When an attachment is provided, body should be optional. At least this is how the command-line works.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] Tests added

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9d3f8c7) 95.23% compared to head (c6e4a57) 95.23%.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #156 +/- ## ======================================= Coverage 95.23% 95.23% ======================================= Files 7 7 Lines 714 714 ======================================= Hits 680 680 Misses 34 34 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.