[Fix] Handle empty/malformed REST API node responses (#255)

TL;DR: the REST API can return valid but malformed responses with empty
data. This small fix defends against that when flattening the response.

Author: jm-rivera

datacommons_client/utils/data_processing.py

@@ -48,7 +48,8 @@ def flatten_properties(
     properties = getattr(node_data, "properties", None)
 
     processed_arcs = unpack_arcs(arcs) if arcs else None
-    items[node] = processed_arcs if processed_arcs is not None else properties
+    if properties or processed_arcs:
+      items[node] = processed_arcs if processed_arcs is not None else properties
 
   return items