make empty header check more explicit

Co-Authored-By: dominicmarmont <[email protected]>

Author: davesque

rest_framework_simplejwt/authentication.py

@@ -66,7 +66,7 @@ def get_raw_token(self, header):
         """
         parts = header.split()
 
-        if not parts:
+        if len(parts) == 0:
             # Empty AUTHORIZATION header sent
             return None