Make Anilist missing username error more reliable, allow editing anilist username in settings panel

src/API/Anilist/Model.php

@@ -100,7 +100,7 @@ final class Model
 		$config = $this->container->get('config');
 		$anilistUser = $config->get(['anilist', 'username']);
 
-		if ( ! \is_string($anilistUser))
+		if ( ! (is_string($anilistUser) && $anilistUser !== ''))
 		{
 			throw new InvalidArgumentException('Anilist username is not defined in config');
 		}

src/constants.php

@@ -61,7 +61,6 @@ const SETTINGS_MAP = [
 			'type' => 'string',
 			'title' => 'Anilist Username',
 			'default' => '',
-			'readonly' => TRUE,
 			'description' => 'Login username for Anilist account to integrate with',
 		],
 		'access_token' => [