Fix performance when JSON is invalid (#10005)

* Fixes https://github.com/flutter/devtools/issues/4172

1. **Fixed performance_model.dart:68-71**:
  * Updated isEmpty to verify that there is no trace binary, no frames, and no rebuild model.

2. **Guarded Perfetto trace loading in timeline_events_controller.dart:501-506**:
  * Avoided attempting to process track events or load a trace into Perfetto when perfettoTraceBinary is null or empty, allowing offline Flutter frames to display cleanly.
3. **Added validation in import_export.dart:72-106**:
  * Added null-safety check for activeScreenId with user-facing notification if missing from the snapshot.
  * Added a check verifying that the JSON contains data for activeScreenId (json.containsKey(activeScreenId) && json[activeScreenId] != null), preventing navigation to a broken snapshot screen and immediately notifying the user.
4. **Added error handling and notifications in offline_data.dart:173-200**:
  * Pushes a notification if the file data for that screen is empty (!shouldLoad(screenData)).
  * Wraps deserialization and loading in a try / catch block to log and display an error message if parsing fails.
5. Tests & Release Notes:
  * Added tests in import_export_test.dart for missing activeScreenId and missing screen data.
  * Added unit tests in performance_model_test.dart for OfflinePerformanceData.isEmpty.
  * Created offline_data_test.dart testing OfflineScreenControllerMixin loading, empty payload notifications, and error handling.
  * Documented the changes in NEXT_RELEASE_NOTES.md.

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