wallet.unlock: raise exception if password is missing (follow-up 2e52b17a43)

electrum/wallet.py

@@ -3108,7 +3108,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
 
     def unlock(self, password):
         self.logger.info(f'unlocking wallet')
-        if password:
+        if self.has_password():
             self.check_password(password)
         self._password_in_memory = password