Update List usage

* Replace [] initialization deprecated to List<MyClass>.empty(growable: true);

Author: Jeasmine

lib/src/create_notification.dart

@@ -172,7 +172,7 @@ class OSCreateNotification extends JSONStringRepresentable {
 
     // adds buttons
     if (this.buttons != null) {
-      var btns = [];
+      var btns = List<Map<String, dynamic>>.empty(growable: true);
       this.buttons!.forEach((btn) => btns.add(btn.mapRepresentation()));
       json['buttons'] = btns;
     }

lib/src/notification.dart

@@ -248,7 +248,7 @@ class OSNotification extends JSONStringRepresentable {
     }
 
     if (json.containsKey('buttons')) {
-      this.buttons = [];
+      this.buttons = List<OSActionButton>.empty(growable: true);
       var btns = json['buttons'] as List<dynamic>;
       for (var btn in btns) {
         var serialized = btn.cast<String, dynamic>();