update json attibute mapping tests

Author: muhammad-othman

sdk/test/Services/DynamoDBv2/UnitTests/Custom/DynamoDBTests.cs

@@ -113,7 +113,9 @@ public void TestConvertingListContainsEmptyMapToJson()
 
             var document = Document.FromAttributeMap(initialAttributeMap);
             var result = document.ToJson();
+            document = Document.FromAttributeMap(document.ToAttributeMap());
 
+            Assert.IsNotNull(document);
             Assert.IsNotNull(result);
             Assert.AreEqual(document["testlist"].AsListOfDocument().Count, 0);
         }
@@ -152,7 +154,9 @@ public void TestJsonContainsEmptyMapToDocumentAndBackToJson(string json)
 
             var document = Document.FromAttributeMap(initialAttributeMap);
             var result = document.ToJson();
+            document = Document.FromAttributeMap(document.ToAttributeMap());
 
+            Assert.IsNotNull(document);
             Assert.IsNotNull(result);
         }