start_scan() needs to default to

Author: dhalbert

adafruit_ble/__init__.py

@@ -230,8 +230,9 @@ def start_scan(
         :rtype: iterable
         """
         prefixes = b""
-        if advertisement_types:
-            prefixes = b"".join(adv.prefix for adv in advertisement_types)
+        if not advertisement_types:
+            advertisement_types = (Advertisement,)
+        prefixes = b"".join(adv.prefix for adv in advertisement_types)
         for entry in self._adapter.start_scan(
             prefixes=prefixes,
             buffer_size=buffer_size,