fix: Null reference exception in Asp.NET Core 6+ browser monitoring injection.

[tippmar-nr]

In some cases, neither of the instrumentation wrappers for Asp.NET Core 6+ will be hit (for example, when OpenAPI and Swagger is configured), which means that no transaction will be created and can result in a null reference exception.

Also updates the pipeline that executes the browser injection wrapper to check for a valid transaction, which will keep the wrapper from running when there is no transaction.

Fixes #3101

[jaffinito]

Smallest PR ever?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.28%. Comparing base (bd9ef4c) to head (2fdc2cb).
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3102      +/-   ##
==========================================
- Coverage   82.33%   82.28%   -0.06%     
==========================================
  Files         477      477              
  Lines       30636    30636              
  Branches     3424     3424              
==========================================
- Hits        25225    25208      -17     
- Misses       4609     4626      +17     
  Partials      802      802              

Flag	Coverage Δ
Agent	83.34% <ø> (-0.07%)	⬇️
Profiler	72.51% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/Agent/NewRelic/Agent/Core/Agent.cs	75.13% <ø> (ø)

... and 4 files with indirect coverage changes

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