fix: use 'is not None' checks for json parameter to preserve falsy values

[vdusek]

Summary

• _prepare_request_call used truthiness checks (if json) to detect JSON payloads
• This silently dropped valid falsy JSON values: 0, "", [], {}, False
• For example, json=[] (empty list) would not be serialized or sent to the API
• Changed both the conflict check and the serialization gate to use is not None

Test plan

• Verify existing unit tests pass
• Verify that http_client.call(..., json=[]) correctly sends an empty JSON array

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.02%. Comparing base (e7ee8b3) to head (a5e3964).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #622   +/-   ##
=======================================
  Coverage   76.02%   76.02%           
=======================================
  Files          42       42           
  Lines        2482     2482           
=======================================
  Hits         1887     1887           
  Misses        595      595           

Flag	Coverage Δ
integration	68.77% <100.00%> (ø)
unit	64.66% <100.00%> (ø)

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[Pijukatel]

I would be worried that maybe we rely on this behavior somewhere...
Could you please add some tests with some values that would be previously ignored and now they would be handled?

[vdusek]

I would be worried that maybe we rely on this behavior somewhere...
Could you please add some tests with some values that would be previously ignored and now they would be handled?

Might be the case. Closing and let's resolve it for the 3.x.