test: align null payload cases with success contract

Author: KSemenenko

ManagedCode.Communication.Tests/Extensions/ResultConversionExtensionsTests.cs

@@ -40,18 +40,11 @@ public void AsResult_WithIntValue_CreatesSuccessfulResult()
     }
 
     [Test]
-    public void AsResult_WithNullValue_CreatesSuccessfulResultWithNull()
+    public void AsResult_WithNullValue_RejectsInvalidSuccess()
     {
-        // Arrange
         string? value = null;
 
-        // Act
-        var result = value.AsResult();
-
-        // Assert
-        result.IsSuccess.ShouldBeTrue();
-        result.Value.ShouldBeNull();
-        result.Problem.ShouldBeNull();
+        Should.Throw<ArgumentNullException>(() => value.AsResult());
     }
 
     [Test]

ManagedCode.Communication.Tests/Orleans/Serialization/ResultSerializationTests.cs

@@ -117,18 +117,16 @@ public async Task ResultT_WithComplexValue_ShouldSerializeCorrectly()
     }
 
     [Test]
-    public async Task ResultT_WithNullValue_ShouldSerializeCorrectly()
+    public async Task ResultT_WithNullFailureValue_ShouldSerializeCorrectly()
     {
-        // Arrange
         var grain = _grainFactory.GetGrain<ITestSerializationGrain>(Guid.NewGuid());
-        var result = Result<string?>.Succeed((string?)null);
+        var result = Result<string?>.Fail();
 
-        // Act
         var echoed = await grain.EchoResultAsync(result);
 
-        // Assert
-        echoed.IsSuccess.ShouldBeTrue();
+        echoed.IsFailed.ShouldBeTrue();
         echoed.Value.ShouldBeNull();
+        echoed.Problem.ShouldNotBeNull();
     }
 
     [Test]