[Fusion] Fixed validation rule "EnumTypeDefaultValueInaccessibleRule" (#8285)

Author: glen-84

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PostMergeValidationRules/EnumTypeDefaultValueInaccessibleRule.cs

@@ -75,7 +75,7 @@ private static bool ValidateDefaultValue(
         switch (defaultValue)
         {
             case EnumValueNode enumValue:
-                var enumType = (MutableEnumTypeDefinition)defaultType;
+                var enumType = (MutableEnumTypeDefinition)defaultType.NullableType();
 
                 if (!enumType.Values.TryGetValue(enumValue.Value, out var value)
                     || value.HasFusionInaccessibleDirective())
@@ -88,7 +88,7 @@ private static bool ValidateDefaultValue(
                 return true;
 
             case ListValueNode listValue:
-                var listType = (ListType)defaultType;
+                var listType = (ListType)defaultType.NullableType();
 
                 foreach (var item in listValue.Items)
                 {
@@ -103,7 +103,7 @@ private static bool ValidateDefaultValue(
                 return true;
 
             case ObjectValueNode objectValue:
-                var inputObjectType = (MutableInputObjectTypeDefinition)defaultType;
+                var inputObjectType = (MutableInputObjectTypeDefinition)defaultType.NullableType();
 
                 foreach (var field in objectValue.Fields)
                 {

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/PostMergeValidationRules/EnumTypeDefaultValueInaccessibleRuleTests.cs

@@ -73,6 +73,28 @@ enum Enum1 {
                     }
                     """
                 ]
+            },
+            // Non-nullable default values.
+            {
+                [
+                    """
+                    # Schema A
+                    type Query {
+                        field1(type: Enum1! = FOO): [Baz!]!
+                        field2(type: [Int!]! = [1]): [Baz!]!
+                        field3(type: Input1! = { field1: 1 }): [Baz!]!
+                    }
+
+                    enum Enum1 {
+                        FOO
+                        BAR
+                    }
+
+                    input Input1 {
+                        field1: Int!
+                    }
+                    """
+                ]
             }
         };
     }