Fix credential hash to return None on empty credentials (#1029)

If discovery is triggered without credentials and discovers devices requiring authentication,
blank credentials are used to initialise the protocols and no connection is actually made.
In this instance we should not return the credentials_hash for blank credentials as it will be invalid.

kasa/tests/fakeprotocol_iot.py

@@ -234,8 +234,8 @@ class FakeIotTransport(BaseTransport):
         return 9999
 
     @property
-    def credentials_hash(self) -> str:
-        return ""
+    def credentials_hash(self) -> None:
+        return None
 
     def set_alias(self, x, child_ids=None):
         if child_ids is None: