Update src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

Author: eiriktsarpalis

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -203,11 +203,7 @@ private static Union ParseUnion(ref Utf8JsonReader reader, JsonSerializerOptions
 
                 Debug.Assert(reader.TokenType is JsonTokenType.PropertyName);
 
-                string? propertyName = reader.GetString();
-                if (propertyName is null)
-                {
-                    throw new JsonException("Property name cannot be null");
-                }
+                string propertyName = reader.GetString()!;
 
                 success = reader.Read();
                 Debug.Assert(success, "custom converters are guaranteed to be passed fully buffered objects");