fix: yield not set calculation error

[j0hannesr0th]

Fixes #1986

Thanks for the video @christianlupus - I haven't knew that I can click "servings" to disable them. When trying to reproduce it I just haven't set it and it was "1" by default.

Please write, what the PR is about and what it does or should do. This does not need to be very long.

none.

Is the PR potentially cause any issues that might be of interest? Any other issues to be tought of?

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• I did check that the app can still be opened and does not throw any browser logs
• I created tests for newly added PHP code (check this if no PHP changes were made)
• I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• I notified the matrix channel if I introduced an API change

[github-actions]

Test Results

   12 files    572 suites   1m 30s ⏱️
  552 tests   552 ✅ 0 💤 0 ❌
2 208 runs  2 207 ✅ 1 💤 0 ❌

Results for commit 407491d.

♻️ This comment has been updated with latest results.

[christianlupus]

LGTM