wp_login_failed not working if only one field is filled out

Hi

I created a login form in the frontend with wp_login_form(). In my functions.php I added an action to prevent the redirect to /wp-login.php if the login failed with add_action('wp_login_failed', …
As it turns out the action only hooks if the username AND password fields are filled in. If one of them is left empty I still get redirected to wp-login.php.

Is there a possibility to check if both fields are filled in to prevent the redirect?

Thanks, Julian