fix: Add more null checks to Asp.NET Core 6+ browser instrumentation logic

[tippmar-nr]

Adds additional null checking in Asp.NET Core 6+ browser instrumentation to prevent null reference exceptions.

Fixes #3069

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.22%. Comparing base (ee04d25) to head (666922b).
⚠️ Report is 117 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3070      +/-   ##
==========================================
+ Coverage   82.18%   82.22%   +0.03%     
==========================================
  Files         477      477              
  Lines       30627    30627              
  Branches     3426     3426              
==========================================
+ Hits        25171    25182      +11     
+ Misses       4656     4644      -12     
- Partials      800      801       +1     

Flag	Coverage Δ
Agent	83.27% <ø> (+0.03%)	⬆️
Profiler	72.51% <ø> (ø)

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see 3 files with indirect coverage changes

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