add call to update and check before classList

[zxwandrew]

• [ X ] briefly describe the changes in this PR
  • fixes Fix Popup.trackPointer Cannot read property 'classList' of undefined errors #8669
  • Add a call to _update when calling trackPointer. Checks to see if this._container exists. If not make sure tracking is set appropriately afterwards in _update. Also make sure the .mapboxgl-popup-track-pointer class is being properly applied in all scenarios. Added some unit test to ensure some new cases will now work.
• [ X ] write tests for all new functionality
• [ X ] manually test the debug page

[andrewharvey]

Great thank you! I also tested this and found it worked well.

[asheemmamoowala]

🎉 Thank you and congratulations on your first contribution @zxwandrew!