self._raw.get('results', []) returns None, not [] which breaks

when we try to iterate it. writing it like this will ensure that the user
will receive an empty response (empty Aggs list) which will not raise
TypeError: 'NoneType' object is not iterable

alpaca_trade_api/polygon/entity.py

@@ -93,7 +93,14 @@ def __init__(self, raw):
         ])
 
     def _raw_results(self):
-        return self._raw.get('results', [])
+        results = self._raw.get('results')
+        if not results:
+            # this is not very pythonic but it's written like this because
+            # the raw response for empty aggs was None, and this:
+            # self._raw.get('results', []) returns None, not [] which breaks
+            # when we try to iterate it.
+            return []
+        return results
 
     def rename_keys(self):
         colmap = {