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/tests/newfakes.py

@@ -305,7 +305,13 @@ class FakeSmartProtocol(SmartProtocol):
 class FakeSmartTransport(BaseTransport):
     def __init__(self, info):
         super().__init__(
-            config=DeviceConfig("127.0.0.123", credentials=Credentials()),
+            config=DeviceConfig(
+                "127.0.0.123",
+                credentials=Credentials(
+                    username="dummy_user",
+                    password="dummy_password",  # noqa: S106
+                ),
+            ),
         )
         self.info = info