Don't write duplicate nulls to trace for object or Uri

Fixes #1120

Src/Newtonsoft.Json.Tests/Serialization/TraceWriterTests.cs

@@ -89,6 +89,51 @@ Newtonsoft.Json Error: 0 : Error!
 ]", traceJsonWriter.GetSerializedJsonMessage());
         }
 
+        [Test]
+        public void WriteNullObject()
+        {
+            StringWriter sw = new StringWriter();
+            TraceJsonWriter traceJsonWriter = new TraceJsonWriter(new JsonTextWriter(sw));
+            traceJsonWriter.WriteStartArray();
+            traceJsonWriter.WriteValue((object)null);
+            traceJsonWriter.WriteEndArray();
+
+            StringAssert.AreEqual(@"Serialized JSON: 
+[
+  null
+]", traceJsonWriter.GetSerializedJsonMessage());
+        }
+
+        [Test]
+        public void WriteNullString()
+        {
+            StringWriter sw = new StringWriter();
+            TraceJsonWriter traceJsonWriter = new TraceJsonWriter(new JsonTextWriter(sw));
+            traceJsonWriter.WriteStartArray();
+            traceJsonWriter.WriteValue((string)null);
+            traceJsonWriter.WriteEndArray();
+
+            StringAssert.AreEqual(@"Serialized JSON: 
+[
+  null
+]", traceJsonWriter.GetSerializedJsonMessage());
+        }
+
+        [Test]
+        public void WriteNullUri()
+        {
+            StringWriter sw = new StringWriter();
+            TraceJsonWriter traceJsonWriter = new TraceJsonWriter(new JsonTextWriter(sw));
+            traceJsonWriter.WriteStartArray();
+            traceJsonWriter.WriteValue((Uri)null);
+            traceJsonWriter.WriteEndArray();
+
+            StringAssert.AreEqual(@"Serialized JSON: 
+[
+  null
+]", traceJsonWriter.GetSerializedJsonMessage());
+        }
+
         [Test]
         public void WriteJRaw()
         {

Src/Newtonsoft.Json/Serialization/TraceJsonWriter.cs

@@ -151,7 +151,14 @@ namespace Newtonsoft.Json.Serialization
             {
                 _textWriter.WriteValue(value);
                 _innerWriter.WriteValue(value);
-                base.WriteValue(value);
+                if (value == null)
+                {
+                    base.WriteUndefined();
+                }
+                else
+                {
+                    base.WriteValue(value);
+                }
             }
         }
 
@@ -201,7 +208,14 @@ namespace Newtonsoft.Json.Serialization
         {
             _textWriter.WriteValue(value);
             _innerWriter.WriteValue(value);
-            base.WriteValue(value);
+            if (value == null)
+            {
+                base.WriteUndefined();
+            }
+            else
+            {
+                base.WriteValue(value);
+            }
         }
 
         public override void WriteValue(ushort value)