Fix JsonWriter.WriteToken to allow null with string token (#2472)

Author: JamesNK

Src/Newtonsoft.Json.Tests/JsonTextWriterTest.cs

@@ -945,13 +945,15 @@ public void WriteTokenDirect()
                 jsonWriter.WriteToken(JsonToken.StartArray);
                 jsonWriter.WriteToken(JsonToken.Integer, 1);
                 jsonWriter.WriteToken(JsonToken.StartObject);
-                jsonWriter.WriteToken(JsonToken.PropertyName, "string");
+                jsonWriter.WriteToken(JsonToken.PropertyName, "integer");
                 jsonWriter.WriteToken(JsonToken.Integer, int.MaxValue);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "null-string");
+                jsonWriter.WriteToken(JsonToken.String, null);
                 jsonWriter.WriteToken(JsonToken.EndObject);
                 jsonWriter.WriteToken(JsonToken.EndArray);
             }
 
-            Assert.AreEqual(@"[1,{""string"":2147483647}]", sb.ToString());
+            Assert.AreEqual(@"[1,{""integer"":2147483647,""null-string"":null}]", sb.ToString());
         }
 
         [Test]

Src/Newtonsoft.Json.Tests/Linq/JTokenWriterTest.cs

@@ -392,5 +392,28 @@ public void DateTimeZoneHandling()
 
             Assert.AreEqual(new DateTime(2000, 1, 1, 1, 1, 1, DateTimeKind.Utc), dt);
         }
+
+        [Test]
+        public void WriteTokenDirect()
+        {
+            JToken token;
+
+            using (JTokenWriter jsonWriter = new JTokenWriter())
+            {
+                jsonWriter.WriteToken(JsonToken.StartArray);
+                jsonWriter.WriteToken(JsonToken.Integer, 1);
+                jsonWriter.WriteToken(JsonToken.StartObject);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "integer");
+                jsonWriter.WriteToken(JsonToken.Integer, int.MaxValue);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "null-string");
+                jsonWriter.WriteToken(JsonToken.String, null);
+                jsonWriter.WriteToken(JsonToken.EndObject);
+                jsonWriter.WriteToken(JsonToken.EndArray);
+
+                token = jsonWriter.Token;
+            }
+
+            Assert.AreEqual(@"[1,{""integer"":2147483647,""null-string"":null}]", token.ToString(Formatting.None));
+        }
     }
 }

Src/Newtonsoft.Json/JsonWriter.cs

@@ -579,8 +579,9 @@ public void WriteToken(JsonToken token, object? value)
                     }
                     break;
                 case JsonToken.String:
-                    ValidationUtils.ArgumentNotNull(value, nameof(value));
-                    WriteValue(value.ToString());
+                    // Allow for a null string. This matches JTokenReader behavior which can read
+                    // a JsonToken.String with a null value.
+                    WriteValue(value?.ToString());
                     break;
                 case JsonToken.Boolean:
                     ValidationUtils.ArgumentNotNull(value, nameof(value));