Fix null property name handling logic in SystemTextJsonValidationMetadataProvider (#47775)

* Fix https://github.com/dotnet/runtime/issues/84830

Fixing what appears to be a negation bug causing null names to be passed to the underlying `JsonNamingPolicy`.

* Update src/Mvc/Mvc.Core/src/ModelBinding/Metadata/SystemTextJsonValidationMetadataProvider.cs

* Update SystemTextJsonValidationMetadataProvider.cs

* Update SystemTextJsonValidationMetadataProvider.cs

* Update src/Mvc/Mvc.Core/src/ModelBinding/Metadata/SystemTextJsonValidationMetadataProvider.cs

Co-authored-by: Safia Abdalla <safia@microsoft.com>

* Add test for null value passed to JsonNamingPolicy

---------

Co-authored-by: Safia Abdalla <safia@microsoft.com>
Co-authored-by: Safia Abdalla <safia@safia.rocks>

Author: eiriktsarpalis

src/Mvc/Mvc.Core/src/ModelBinding/Metadata/SystemTextJsonValidationMetadataProvider.cs

@@ -57,7 +57,9 @@ public void CreateValidationMetadata(ValidationMetadataProviderContext context)
 
         if (string.IsNullOrEmpty(propertyName))
         {
-            propertyName = _jsonNamingPolicy.ConvertName(context.Key.Name!);
+            propertyName = context.Key.Name is string contextKeyName
+                ? _jsonNamingPolicy.ConvertName(contextKeyName)
+                : null;
         }
 
         context.ValidationMetadata.ValidationModelName = propertyName;

src/Mvc/Mvc.Core/test/ModelBinding/Metadata/SystemTextJsonValidationMetadataProviderTest.cs

@@ -44,6 +44,22 @@ public void CreateValidationMetadata_SetValidationPropertyName_CamelCaseWithDefa
         Assert.Equal(JsonNamingPolicy.CamelCase.ConvertName(propertyName), context.ValidationMetadata.ValidationModelName);
     }
 
+    [Fact]
+    // Test for https://github.com/dotnet/aspnetcore/issues/47835
+    public void CreateValidationMetadata_SetValidationPropertyName_WithNullKeyName()
+    {
+        var metadataProvider = new SystemTextJsonValidationMetadataProvider(JsonNamingPolicy.SnakeCaseLower);
+        var key = ModelMetadataIdentity.ForType(typeof(SampleTestClass));
+        var modelAttributes = new ModelAttributes(Array.Empty<object>(), Array.Empty<object>(), Array.Empty<object>());
+        var context = new ValidationMetadataProviderContext(key, modelAttributes);
+
+        // Act
+        metadataProvider.CreateValidationMetadata(context);
+
+        // Assert
+        Assert.Null(context.ValidationMetadata.ValidationModelName);
+    }
+
     [Theory]
     [MemberData(nameof(NamingPolicies))]
     public void CreateValidationMetadata_SetValidationPropertyName_WithJsonNamingPolicy(JsonNamingPolicy namingPolicy)