fix: android side get reactions of message null pointer error

[flowkscai]

问题描述：

安卓下获取消息 Reaction 列表抛空指针异常

问题定位：

EMMessageWrapper.java

private void reactionList(JSONObject params, String channelName, MethodChannel.Result result) throws JSONException {
      String msgId = params.getString("msgId");
      EMMessage msg = getMessageWithId(msgId);
      ArrayList<Map<String, Object>> list = new ArrayList<>();
      if (msg != null) {
          List<EMMessageReaction> reactions = msg.getMessageReaction();  // <== 可能返回 null
          for (int i = 0; i < reactions.size(); i++) {
              list.add(EMMessageReactionHelper.toJson(reactions.get(i)));
          }
      }
      onSuccess(result, channelName, list);
  }