bug(web): `TypeError: Cannot read properties of undefined (reading 'insert')` in `TransformUtils.isEmpty`

This appears to be related to whitespace issues in the ait.mnw.mon model.

Captured when I pressed spacebar. This is on an internal local build, but does not appear to be related to my changes on the local build. Not reported to Sentry -- perhaps because it's in the worker?

```
worker-thread-ait.mnw.mon.model.js:1895 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'insert')
at TransformUtils.isEmpty (worker-thread-ait.mn…on.model.js:1895:24)
at ContextTokenization.mapWhitespacedTokenization (worker-thread-ait.mn…n.model.js:4607:107)
at precomputeTransitions (worker-thread-ait.mn…on.model.js:5234:55)
at ContextState.analyzeTransition (worker-thread-ait.mn…on.model.js:5551:71)
at determineContextTransition (worker-thread-ait.mn…on.model.js:5703:37)
at worker-thread-ait.mn…on.model.js:5790:26
at Generator.next ()
at worker-thread-ait.mnw.mon.model.js:46:63
at new Promise ()
at __async (worker-thread-ait.mnw.mon.model.js:30:12)
```

Reproduce the error by pressing the spacebar with the ait.mnw.mon model and inspect the worker stack at TransformUtils.isEmpty, ContextTokenization.mapWhitespacedTokenization, and precomputeTransitions. Trace why the value used by isEmpty is undefined; done means the transition completes without the reported TypeError.

Written by the indexing model from the issue text.