Fix RequestDelegateFactory to handle nullable types correctly in empty request body scenarios

Author: opuzakov

src/Http/Http.Extensions/src/RequestDelegateFactory.cs

@@ -1375,7 +1375,9 @@ private static Expression AddResponseWritingToMethodCall(Expression methodCall,
         {
             object? defaultBodyValue = null;
 
-            if (allowEmptyRequestBody && bodyType.IsValueType)
+            var isNullableType = bodyType.IsGenericType && bodyType.GetGenericTypeDefinition() == typeof(Nullable<>);
+
+            if (allowEmptyRequestBody && bodyType.IsValueType && !isNullableType) // Nullable types should be left null
             {
                 defaultBodyValue = CreateValueType(bodyType);
             }

src/Http/Http.Extensions/test/RequestDelegateFactoryTests.cs

@@ -896,6 +896,27 @@ public async Task RequestDelegateRejectsEmptyBodyGivenExplicitFromBodyParameter(
         Assert.Equal(400, httpContext.Response.StatusCode);
     }
 
+    [Fact]
+    public async Task RequestDelegatePopulatesNullFromBodyNullableParameterWithEmptyBody()
+    {
+        var httpContext = CreateHttpContext();
+        httpContext.Request.Headers["Content-Type"] = "application/json";
+        httpContext.Request.Headers["Content-Length"] = "0";
+        httpContext.Features.Set<IHttpRequestBodyDetectionFeature>(new RequestBodyDetectionFeature(false));
+
+        var factoryResult = RequestDelegateFactory.Create(
+            ([FromBody] int? body) =>
+            {
+                httpContext.Items["body"] = body;
+            }
+        );
+
+        var requestDelegate = factoryResult.RequestDelegate;
+        await requestDelegate(httpContext);
+
+        Assert.Null(httpContext.Items["body"]);
+    }
+
     [Fact]
     public void RequestDelegateFactoryThrowsForByRefReturnTypes()
     {