Add .structure() to mongo_normalized adapters

This is now required by Tiled since v0.1.0a117

databroker/mongo_normalized.py

@@ -643,6 +643,9 @@ def __init__(
     def metadata(self):
         return self._metadata
 
+    def structure(self):
+        return None
+
     def keys(self):
         return self._contents.keys()
 
@@ -1418,6 +1421,9 @@ def metadata(self):
         "Metadata about this MongoAdapter."
         return self._metadata
 
+    def structure(self):
+        return None
+
     @property
     def sorting(self):
         return list(self._sorting)