fix: error handling on identity import

[zepatrik]

When importing identities without any traits, or with malformed traits, 500s are returned. This improves the error handling and messaging.
Per default, missing traits now get rewritten to an empty object. Otherwise sjson would produce invalid JSON, and JSON parse errors would be returned even though the payload was valid.

[codecov]

Codecov Report

Merging #3520 (6043b68) into master (32d8306) will increase coverage by 0.13%.
The diff coverage is 76.92%.

❗ Current head 6043b68 differs from pull request most recent head 9372466. Consider uploading reports for the commit 9372466 to get more accurate results

@@            Coverage Diff             @@
##           master    #3520      +/-   ##
==========================================
+ Coverage   78.17%   78.31%   +0.13%     
==========================================
  Files         341      341              
  Lines       22754    22679      -75     
==========================================
- Hits        17789    17761      -28     
+ Misses       3626     3591      -35     
+ Partials     1339     1327      -12     

Files Changed	Coverage Δ
schema/validator.go	85.71% <50.00%> (-6.29%)	⬇️
identity/validator.go	78.57% <83.33%> (+3.57%)	⬆️
identity/handler.go	85.32% <100.00%> (ø)
identity/manager.go	75.22% <100.00%> (+0.11%)	⬆️

... and 19 files with indirect coverage changes

[zepatrik]

This is the fix. Without, sjson.SetRawBytes below would produce invalid JSON, although the payload is valid JSON.