update test code

Author: Simon Zhao (BEYONDSOFT CONSULTING INC)

test/dotnet.Tests/GivenExponentialRetry.cs

@@ -29,12 +29,12 @@ public async Task ItReturnsOnSuccess()
         public async Task ItRetriesOnError()
         {
             var retryCount = 0;
-            Func<Task<string>> action = () =>
+            Func<Task<string?>> action = () => // Updated to use nullable reference type  
             {
                 retryCount++;
-                return Task.FromResult<string>(null);
+                return Task.FromResult<string?>(null); // Updated to match nullable reference type  
             };
-            var res = await ExponentialRetry.ExecuteWithRetryOnFailure<string>(action, 2, timer: () => ExponentialRetry.Timer(ExponentialRetry.TestingIntervals));
+            var res = await ExponentialRetry.ExecuteWithRetryOnFailure<string?>(action, 2, timer: () => ExponentialRetry.Timer(ExponentialRetry.TestingIntervals));
 
             res.Should().BeNull();
             retryCount.Should().Be(2);