Leaving account_id or region parameter empty causes incorrect AWS configuration for some actions. #104

While using the invoke_lambda action on StackStorm 3.1.0 on Python 3.6.9 I encounter a bug with the following lines:

Debugging has shown that actions which do not specify an account_id or a region are still receiving them in the kwargs dict, but with a None value.

I would recommend this change:

account_id = kwargs.pop('account_id', None)
if account_id:
    self.assume_role(account_id)

region = kwargs.pop('region', None)
if region:
    self.credentials['region'] = region