Instantiation check: Improve reaction if no model answers

Currently, if no model answers have been defined, the variable instantiation check in the edit form will fail with a notification:

<img width="459" alt="Image" src="https://github.com/user-attachments/assets/42c078e8-b8b0-4b8b-b4c4-fb7cdbe20742" />

We should anticipate that and display a nicer notice below the table.