check if bucket exists in usage endpoint

app/controllers/api/storage.php

@@ -1259,7 +1259,11 @@ App::get('/v1/storage/:bucketId/usage')
         /** @var Appwrite\Utopia\Response $response */
         /** @var Utopia\Database\Database $dbForInternal */
 
-        // TODO: Check if the storage bucket exists else throw 404 
+        $bucket = $dbForInternal->getDocument('buckets', $bucketId);
+
+        if($bucket->isEmpty()) {
+            throw new Exception('Bucket not found', 404);
+        } 
         
         $usage = [];
         if (App::getEnv('_APP_USAGE_STATS', 'enabled') == 'enabled') {