Cleanup credentials handling (#605)

* credentials: don't allow none to simplify checks

* Implement __bool__ for credentials

* Cleanup klaptransport cred usage

* Cleanup deviceconfig and tapodevice

* fix linting

* Pass dummy credentials for tests

* Remove __bool__ dunder and add docs to credentials

* Check for cred noneness in tapodevice.update()

kasa/exceptions.py

@@ -26,6 +26,7 @@ class AuthenticationException(SmartDeviceException):
 class RetryableException(SmartDeviceException):
     """Retryable exception for device errors."""
 
+
 class TimeoutException(SmartDeviceException):
     """Timeout exception for device errors."""