`buildQueryLines` should use pattern matching rather than null assertions

The generated code would change like:
```diff
-      if (request.filter != null) 'filter': request.filter!,
+      if (request.filter case final filter?) 'filter': filter,
```

See https://github.com/googleapis/librarian/blob/bcb914ac67afc3df0e28483e20dcc9232b42e1c5/internal/sidekick/internal/dart/annotate.go#L789

