[Internal] Indexing Policy: Fixes upgrade scenario to ignore Annotation property if null (#2310)

Co-authored-by: j82w <[email protected]>

Author: bemill

Microsoft.Azure.Cosmos/src/Resource/Settings/IndexingPolicy.cs

@@ -120,7 +120,7 @@ public IndexingPolicy()
         /// <summary>
         /// Indexing policy annotation.
         /// </summary>
-        [JsonProperty(PropertyName = "annotation")]
+        [JsonProperty(PropertyName = "annotation", NullValueHandling = NullValueHandling.Ignore)]
         public string Annotation { get; set; }
 #endif