Set default key to empty str

ref: https://github.com/jpadilla/pyjwt/blob/c4973c7dd602eac984877ceb0186ef5f233bdb8f/jwt/api_jwt.py#L114

Author: q0w

rest_framework_simplejwt/backends.py

@@ -20,7 +20,7 @@ def __init__(
         self,
         algorithm,
         signing_key=None,
-        verifying_key=None,
+        verifying_key="",
         audience=None,
         issuer=None,
         jwk_url: str = None,