Correctly check for a running transaction

[antonpirker]

Fix check for the existence of a currently transaction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.53%. Comparing base (ca12bbf) to head (b94f503).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4791      +/-   ##
==========================================
- Coverage   84.54%   84.53%   -0.02%     
==========================================
  Files         158      158              
  Lines       16490    16490              
  Branches     2856     2856              
==========================================
- Hits        13942    13940       -2     
  Misses       1702     1702              
- Partials      846      848       +2     

Files with missing lines	Coverage Δ
sentry_sdk/ai/utils.py	77.77% <ø> (ø)

... and 2 files with indirect coverage changes