Fix coding style issue

src/data/entity_manager.rs

@@ -30,6 +30,10 @@ impl<E> EntityManager<E> where E: rocket::serde::Serialize + rocket::serde::Dese
         self.list.len()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Save the list
     fn save(&self) -> Res {
         Ok(std::fs::write(&self.file_path, serde_json::to_string(&self.list)?)?)