Anonymous login requires account to be set

To enable anonymous login, we need both allow_anonymous_login and the
anonymous_account to be set.

The former without the latter results in a MantisBT generic error when
trying to login anonymously, as login.php is called with an empty
username.

Fixes #25061

core/authentication_api.php

@@ -176,7 +176,7 @@ function auth_signup_access_level() {
  * @return bool true: enabled; false: otherwise.
  */
 function auth_anonymous_enabled() {
-	return config_get_global( 'allow_anonymous_login' );
+	return config_get_global( 'allow_anonymous_login' ) && auth_anonymous_account();
 }
 
 /**