Fixes how arg values will be handled when the input obj is inferred

src/HotChocolate/Core/src/Types.Mutations/MutationConventionMiddleware.cs

@@ -35,27 +35,32 @@ public async ValueTask InvokeAsync(IMiddlewareContext context)
 
         var arguments = new Dictionary<string, ArgumentValue>(StringComparer.Ordinal);
         var preservedArguments = context.ReplaceArguments(arguments);
-        var inputArgument = preservedArguments[_inputArgumentName];
 
         foreach (var argument in _resolverArguments)
         {
             input.TryGetValue(argument.Name, out var value);
 
-            inputLiteral.TryGetValue(argument.Name, out var valueLiteral);
+            var omitted = false;
+            if (!inputLiteral.TryGetValue(argument.Name, out var valueLiteral))
+            {
+                omitted = true;
+                valueLiteral = argument.DefaultValue;
+                value = null;
+            }
             valueLiteral ??= NullValueNode.Default;
 
             if (!valueLiteral.TryGetValueKind(out var kind))
             {
                 kind = ValueKind.Unknown;
             }
-
+            
             arguments.Add(
                 argument.Name,
                 new ArgumentValue(
                     argument,
                     kind,
-                    true,
-                    inputArgument.IsDefaultValue,
+                    !omitted,
+                    omitted,
                     value,
                     valueLiteral));
         }

src/HotChocolate/Core/test/Types.Mutations.Tests/AnnotationBasedMutations.cs

@@ -1137,8 +1137,37 @@ public async Task Mutation_Aggregate_Error_Not_Mapped()
 
         result.MatchSnapshot();
     }
+
+    [Fact]
+    public async Task Mutation_With_Optional_Arg()
+    {
+        var result =
+            await new ServiceCollection()
+                .AddGraphQL()
+                .AddQueryType(d => d.Field("abc").Resolve("def"))
+                .AddMutationType<MutationWithOptionalArg>()
+                .AddMutationConventions()
+                .ExecuteRequestAsync(
+                    """
+                    mutation {
+                        doSomething(input: { }) {
+                            string
+                        }
+                    }
+                    """);
 
-
+        result.MatchInlineSnapshot(
+            """
+            {
+              "data": {
+                "doSomething": {
+                  "string": "nothing"
+                }
+              }
+            }
+            """);
+    }
+
     public class SimpleMutation
     {
         public string DoSomething(string something)
@@ -1531,4 +1560,10 @@ public override void OnConfigure(IDescriptorContext context, ObjectFieldDefiniti
                 }));
         }
     }
+
+    public class MutationWithOptionalArg
+    {
+        public string DoSomething(Optional<string?> something)
+            => something.Value ?? "nothing";
+    }
 }