fix(simmy): return null from FaultGenerator when no fault is generated

FaultGenerator's implicit conversion dereferenced the nullable Outcome
returned by the underlying generator via `!.Value`, throwing
InvalidOperationException ("Nullable object must have a value.") when the
generator produced no outcome - an empty generator, or registered weights
summing to zero. ChaosFaultStrategy treats a null fault as "do not inject"
and only catches OperationCanceledException, so the exception escaped and
failed the execution the strategy was meant to leave untouched.

Propagate the nullable outcome with `?.Exception`, mirroring
OutcomeGenerator<TResult>, so no fault is injected when none is generated.

test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs

@@ -39,6 +39,24 @@ public class FaultGeneratorTests
         Generate(generator).ShouldBeOfType<InvalidOperationException>();
     }
 
+    [Fact]
+    public void NoExceptionRegistered_ShouldReturnNull()
+    {
+        var generator = new FaultGenerator();
+
+        Generate(generator).ShouldBeNull();
+    }
+
+    [Fact]
+    public void AllWeightsZero_ShouldReturnNull()
+    {
+        var generator = new FaultGenerator();
+
+        generator.AddException<InvalidOperationException>(weight: 0);
+
+        Generate(generator).ShouldBeNull();
+    }
+
     private static Exception? Generate(FaultGenerator generator)
     {
         Func<FaultGeneratorArguments, ValueTask<Exception?>> func = generator;