Fix reading null struct property value in gRPC JSON transcoding (#52442)

Author: JamesNK

src/Grpc/JsonTranscoding/src/Microsoft.AspNetCore.Grpc.JsonTranscoding/Internal/Json/ValueConverter.cs

@@ -10,6 +10,8 @@ namespace Microsoft.AspNetCore.Grpc.JsonTranscoding.Internal.Json;
 
 internal sealed class ValueConverter<TMessage> : SettingsConverterBase<TMessage> where TMessage : IMessage, new()
 {
+    public override bool HandleNull => true;
+
     public ValueConverter(JsonContext context) : base(context)
     {
     }

src/Grpc/JsonTranscoding/test/Microsoft.AspNetCore.Grpc.JsonTranscoding.Tests/ConverterTests/JsonConverterReadTests.cs

@@ -120,6 +120,46 @@ public void RepeatedStrings()
         AssertReadJson<HelloRequest>(json);
     }
 
+    [Fact]
+    public void Struct_NullProperty()
+    {
+        var json = @"{ ""prop"": null }";
+
+        AssertReadJson<Struct>(json);
+    }
+
+    [Fact]
+    public void Value_Null()
+    {
+        var json = "null";
+
+        AssertReadJson<Value>(json);
+    }
+
+    [Fact]
+    public void Value_Integer()
+    {
+        var json = "1";
+
+        AssertReadJson<Value>(json);
+    }
+
+    [Fact]
+    public void Value_String()
+    {
+        var json = @"""string!""";
+
+        AssertReadJson<Value>(json);
+    }
+
+    [Fact]
+    public void Value_Boolean()
+    {
+        var json = "true";
+
+        AssertReadJson<Value>(json);
+    }
+
     [Fact]
     public void DataTypes_DefaultValues()
     {

src/Grpc/JsonTranscoding/test/Microsoft.AspNetCore.Grpc.JsonTranscoding.Tests/ConverterTests/JsonConverterWriteTests.cs

@@ -334,6 +334,23 @@ public void Value_Nested()
         AssertWrittenJson(helloRequest);
     }
 
+    [Fact]
+    public void Struct_NullValue()
+    {
+        var helloRequest = new HelloRequest
+        {
+            ValueValue = Value.ForStruct(new Struct
+            {
+                Fields =
+                {
+                    ["prop"] = Value.ForNull()
+                }
+            })
+        };
+
+        AssertWrittenJson(helloRequest);
+    }
+
     [Fact]
     public void Value_Root()
     {
@@ -351,6 +368,14 @@ public void Value_Root()
         AssertWrittenJson(value);
     }
 
+    [Fact]
+    public void Value_Null()
+    {
+        var value = Value.ForNull();
+
+        AssertWrittenJson(value);
+    }
+
     [Fact]
     public void Struct_Nested()
     {