Fix base attributes are not set (#7)

Author: pvizeli

pycognito/__init__.py

@@ -323,9 +323,13 @@ def register(self, username, password, attr_map=None):
             }
         }
         """
-        attributes = self.base_attributes.copy()
+        if self.base_attributes is None:
+            attributes = {}
+        else:
+            attributes = self.base_attributes.copy()
         if self.custom_attributes:
             attributes.update(self.custom_attributes)
+
         cognito_attributes = dict_to_cognito(attributes, attr_map)
         params = {
             "ClientId": self.client_id,