[MOB-19790] Fix encoded data in migration

[cdhoffmann]

Fixes crash which happened when trying to set Data in FileSystemNamedCollection.

The crash occurred because the JSONSerialization.data(:_) method crashes internally when a top level fragment is not an accepted type. See here. When we attempted to set encoded data, the crash occurs.

To avoid the crash, we use the JSONSerialization.isValidJSONObject to check if the value can be used.
To successfully migrate the encoded data, we now encode the data as a string in the migrator before migrating it.

[codecov]

Codecov Report

Merging #974 (bbfb306) into dev-v4.2.0 (9dad765) will decrease coverage by 0.05%.
The diff coverage is 84.62%.

@@              Coverage Diff               @@
##           dev-v4.2.0     #974      +/-   ##
==============================================
- Coverage       89.91%   89.85%   -0.05%     
==============================================
  Files             133      133              
  Lines            8331     8338       +7     
==============================================
+ Hits             7490     7492       +2     
- Misses            841      846       +5     

[kevinlind]

Looks good!
Were you able to test with Core's test app? Did you see any cases where the migrated data was not serializable?

[cdhoffmann]

@kevinlind Just tested on the core test app and the issue was that we were setting the app group on the dev branch but not on main. I went ahead and removed that from our dev branch to avoid issues.