Allow transcriptions on elements with no zones but children

We are creating more & more complex hierarchy, now aggregating zones on images on "virtual" elements that are not tied to any images.

We need to create transcriptions on these images: this is allowed by the database (no constraint), but not allowed by the CreateTranscription endpoint.

The endpoint should have a third condition that would allow to create transcriptions on elements with children (themselves having a transcription).

I think it's important to check that the children have a transcription to avoid opening a hole too large...

⚠ Ideally this condition will be checked as a last resort, as it's a lot more compute intensive than checking if the element has a zone or polygon.