Return null from FaultGenerator when no fault is generated

[dualfroz]

The issue or feature being addressed

FaultGenerator throws InvalidOperationException ("Nullable object must have a
value.") instead of injecting no fault when its underlying generator yields no
outcome.

The implicit conversion operator on FaultGenerator
(src/Polly.Core/Simmy/Fault/FaultGenerator.cs) builds its delegate as:

return args => new ValueTask<Exception?>(generatorDelegate(args.Context)!.Value.Exception);

generatorDelegate is a Func<ResilienceContext, Outcome<VoidResult>?> produced by
GeneratorHelper<VoidResult>.CreateGenerator(). It legitimately returns null when:

• no exception has been registered (an empty FaultGenerator), in which case
  CreateGenerator() returns _ => null; or
• the registered weights sum to zero (for example AddException<T>(weight: 0)), in
  which case the selection loop matches no factory and returns null.

Because Outcome<TResult> is a readonly struct, Outcome<VoidResult>? is a
Nullable<Outcome<VoidResult>>. The null-forgiving ! operator has no runtime effect,
so the subsequent .Value dereferences an empty nullable and throws
InvalidOperationException.

ChaosFaultStrategy.ExecuteCore explicitly treats a null fault as "do not inject"
(if (fault is not null)), and its try block only catches
OperationCanceledException, so the exception escapes and fails the user's execution
that the strategy was meant to leave untouched.

The sibling OutcomeGenerator<TResult> handles the same situation correctly by
propagating the nullable value unchanged
(src/Polly.Core/Simmy/Outcomes/OutcomeGenerator.cs):

return args => new ValueTask<Outcome<TResult>?>(generatorDelegate(args.Context));

Details on the issue fix or feature implementation

Propagate null from the generator instead of dereferencing it, mirroring
OutcomeGenerator<TResult>:

return args => new ValueTask<Exception?>(generatorDelegate(args.Context)?.Exception);

When a fault is produced, Outcome<VoidResult>.Exception returns the same exception as
before; when the generator yields no outcome, the delegate now returns null and
ChaosFaultStrategy proceeds without injecting a fault.

Added two unit tests to test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs:

• NoExceptionRegistered_ShouldReturnNull - an empty FaultGenerator returns null.
• AllWeightsZero_ShouldReturnNull - a generator whose weights sum to zero returns null.

Both tests fail on the unmodified code with
System.InvalidOperationException : Nullable object must have a value. at
FaultGenerator.cs:81, and pass after the fix.

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
• I have successfully run a local build

[dualfroz]

@dotnet-policy-service agree

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.16%. Comparing base (df53959) to head (65d060c).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3220   +/-   ##
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  Coverage   96.16%   96.16%           
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  Files         311      311           
  Lines        7142     7142           
  Branches     1006     1007    +1     
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  Hits         6868     6868           
  Misses        221      221           
  Partials       53       53           

Flag	Coverage Δ
linux	96.16% <100.00%> (ø)
macos	96.16% <100.00%> (ø)
windows	96.15% <100.00%> (ø)

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[github-actions]

Thanks for your contribution @dualfroz - the changes from this pull request have been published as part of version 8.8.0 📦, which is now available from NuGet.org 🚀