Fix warning CA1062#ContextualTtl

[Zombach]

Pull Request

The issue or feature being addressed

#2215

Details on the issue fix or feature implementation

• Suppress CA1062 in src/Polly/Caching/ContextualTtl.cs or fix the warning

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature

[Zombach]

@martincostello
Which is better to use? == or the pattern is
The is mapping pattern, but mostly the code uses ==

[martincostello]

I would use is for new code where it is equivalent to == null. Don't worry about updating existing code to match - it was written before is was a thing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.90%. Comparing base (e4ffcd4) to head (ccfc752).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2217   +/-   ##
=======================================
  Coverage   83.89%   83.90%           
=======================================
  Files         313      313           
  Lines        7179     7181    +2     
  Branches     1060     1061    +1     
=======================================
+ Hits         6023     6025    +2     
  Misses        785      785           
  Partials      371      371           

Flag	Coverage Δ
linux	83.90% <100.00%> (+<0.01%)	⬆️
macos	83.90% <100.00%> (+<0.01%)	⬆️
windows	?

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