Strict null check 'completionModel.ts'

[mjbvz]

Part of #60565

Tracks updating ./vs/editor/contrib/suggest/completionModel.ts to have strict null checking

Steps

1. Add ./vs/editor/contrib/suggest/completionModel.ts to the files section of src/tsconfig.strictNullChecks.json
2. Run yarn strict-null-check -- --watch
3. Fix strict null check compiler errors
4. Verify normal build still works