Add empty tags by default to transaction

[sl0thentr0py]

Seems like some before_sends out there might assume tags without safe access as we found out during dogfooding.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.97%. Comparing base (53005b9) to head (5075d22).
⚠️ Report is 3 commits behind head on potel-base.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4683      +/-   ##
==============================================
- Coverage       85.01%   84.97%   -0.05%     
==============================================
  Files             158      158              
  Lines           15540    15541       +1     
  Branches         2461     2461              
==============================================
- Hits            13212    13206       -6     
- Misses           1572     1579       +7     
  Partials          756      756              

Files with missing lines	Coverage Δ
sentry_sdk/opentelemetry/span_processor.py	82.65% <100.00%> (+0.10%)	⬆️

... and 2 files with indirect coverage changes