Change serialization of SyncFrom to NULL

I changed the serialization of the SynFrom property from Javascript's
undefined to null as required by the API. Without it new synchronized
blocks cannot be created.

Author: taco-indenbosch

Src/Notion.Client/Models/Blocks/Request/SyncedBlockBlockRequest.cs

@@ -12,7 +12,7 @@ public class SyncedBlockBlockRequest : BlockObjectRequest, IColumnChildrenBlockR
 
         public class Data
         {
-            [JsonProperty("synced_from")]
+            [JsonProperty("synced_from", NullValueHandling = NullValueHandling.Include)]
             public SyncedFromBlockId SyncedFrom { get; set; }
 
             [JsonProperty("children")]