Don't serialise empty content on ChatCompletionMessage

[greysteil]

Describe the change

Updates the JSON serialisation for ChatCompletionMessage structs to omit empty Content. This avoids a bug in the OpenAI API where sending a message that has both a tool call and empty content returns a 500.

Provide OpenAI documentation link

https://platform.openai.com/docs/api-reference/chat/object#chat/object-choices

Describe your solution

Since the content field can be a string or null, representing it as a string is technically wrong. Changing that would be a breaking change, but we can at least ensure that empty strings (which are unlikely to ever serve any purpose) aren't sent to OpenAI

Tests

Added an integration test. If my change is removed OpenAI returns a 500.

Additional context

I'll also report the 500s to OpenAI.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.02%. Comparing base (774fc9d) to head (4381429).
Report is 46 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
+ Coverage   98.46%   99.02%   +0.55%     
==========================================
  Files          24       26       +2     
  Lines        1364     1429      +65     
==========================================
+ Hits         1343     1415      +72     
+ Misses         15        8       -7     
  Partials        6        6              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.