Fix 500 on panorama building save (for real this time)

[WillNilges]

Finally managed to reproduce this locally (I believe with 293 Bainbridge St, which has no panos: (http://127.0.0.1:8000/admin/meshapi/building/0031a0dc-505d-4ba8-a124-170891c99bc7/change/))

So I added a guard to the JSON parsing logic that should fix this. I need to see if I can add a test, and codecov agrees T_T.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.03%. Comparing base (912984a) to head (436a447). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #622 +/- ## ========================================== + Coverage 94.02% 94.03% +0.01% ========================================== Files 75 75 Lines 3081 3087 +6 ========================================== + Hits 2897 2903 +6 Misses 184 184 ```

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