Added null check on raw JSON to prevent null reference error deserializing the grid

(cherry picked from commit bcea0f645a)

src/Umbraco.Web/PropertyEditors/GridPropertyEditor.cs

@@ -165,6 +165,10 @@ namespace Umbraco.Web.PropertyEditors
             public IEnumerable<UmbracoEntityReference> GetReferences(object value)
             {
                 var rawJson = value == null ? string.Empty : value is string str ? str : value.ToString();
+
+                if (rawJson.IsNullOrWhiteSpace())
+                    yield break;
+
                 DeserializeGridValue(rawJson, out var richTextEditorValues, out var mediaValues);
 
                 foreach (var umbracoEntityReference in richTextEditorValues.SelectMany(x =>