refactor: Add null-handling to tracer

[philprime]

This PR is derived from #5572 in an effort to make the large amount of changes easier to review for #5577.

Resolves warnings in tracing related files.

#skip-changelog

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3c86ff3). Learn more about missing BASE report.
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
Sources/Sentry/SentryTraceContext.m	80.000%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #5888   +/-   ##
========================================
  Coverage        ?   86.666%           
========================================
  Files           ?       423           
  Lines           ?     36533           
  Branches        ?     17284           
========================================
  Hits            ?     31662           
  Misses          ?      4825           
  Partials        ?        46           

Files with missing lines	Coverage Δ
Sources/Sentry/SentryTracer.m	97.704% <ø> (ø)
Sources/Sentry/SentryTraceContext.m	95.953% <80.000%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3c86ff3...883f2fc. Read the comment docs.

[philipphofmann]

Thanks