Predictions from imported tasks cannot be deleted

**Describe the bug**
When pre-annotationed tasks are imported to Label Studio, via the cloud storage interface, `model_version` is not a required field on the JSON files. This make it possible to create predication with no model version information attached, and this then causes a error, if you try to delete those predictions.

**To Reproduce**
Steps to reproduce the behavior:
1. Import pre-annotationed tasks from a JSON file without a `model_version` field
2. Go to Predictions tab in the project setting
3. Try to delete the predictions

A runtime error is shown:
> Validation error
> model_version param is required
> Version: 1.23.0

**Expected behavior**
Either it should be possible to delete predictions without a model version, or it should be impossible to import tasks without a `model_version` field.

**Screenshots**

**Environment:**
- OS: Ubuntu 22.04.5 LTS
- Label Studio Version 1.23