Prevent Thrift errors from successfully generating schemas

See a recent run where we had an undefined value:
- Android https://github.com/guardian/bridget/actions/runs/8633992774/job/23668359580#step:4:1691
- iOS did not even have an error… https://github.com/guardian/bridget/actions/runs/8633992774/job/23668359871
- NPM’s error is different: https://github.com/guardian/bridget/actions/runs/8633992774/job/23668359259#step:5:10

We should fail earlier if the model is incorrect!

Start by reproducing the undefined-value case in the linked Android, iOS, and NPM Actions runs, then trace the schema-generation entry point that handles the model. Done means an incorrect model fails before any schema is successfully generated on each affected platform.

Written by the indexing model from the issue text.