Fix initialization bug: FbxPrefab string may be null.

That caused an exception to be thrown when you manually created an FbxPrefab.

Author: Benoit Hudson

Assets/FbxExporters/FbxPrefab.cs

@@ -295,7 +295,7 @@ public FbxRepresentation(string json, ref int index) {
             }
 
             public FbxRepresentation(string json) {
-                if (json.Length == 0) { return; }
+                if (string.IsNullOrEmpty(json)) { return; }
                 int index = 0;
                 InitFromJson(json, ref index);
             }