[Instrumentation.WCF] Fix null handling for AsyncCallback parameters

[ysolomchenko]

Fixes # N/A
Design discussion issue # N/A

Found by Codex analysis

Changes

Fixed an issue where instrumented WCF channel Begin* methods threw an ArgumentNullException when AsyncCallback was null.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.91%. Comparing base (b4c4b44) to head (10c5322).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4164      +/-   ##
==========================================
+ Coverage   73.74%   73.91%   +0.17%     
==========================================
  Files         466      458       -8     
  Lines       18314    18263      -51     
==========================================
- Hits        13506    13500       -6     
+ Misses       4808     4763      -45     

Flag	Coverage Δ
unittests-Instrumentation.Cassandra	?
unittests-Instrumentation.Wcf	80.77% <100.00%> (+1.09%)	⬆️

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

Files with missing lines	Coverage Δ
...cf/Implementation/AsyncResultWithTelemetryState.cs	84.61% <100.00%> (-1.10%)	⬇️
...on.Wcf/Implementation/InstrumentedDuplexChannel.cs	65.78% <ø> (+6.03%)	⬆️
...n.Wcf/Implementation/InstrumentedRequestChannel.cs	73.91% <ø> (+5.16%)	⬆️

... and 8 files with indirect coverage changes

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