You have to use the logic when web got result. Just see the response if value found or not found. Calculate the length of the list. And use your logic
If no result found mean see what your app returned. Either empty list or not. If it is empty list mean imagine no result. So you feel now simple to design
You won't know if you have any matches until the the result of the query is returned from the sheet. Therefore, as Spicy Topics says, you can only do the test (matches/no matches) in the Web.Gottext event once you have the result.
I don't believe you learnt the lesson from previous posts. Your query will return a list of lists - [["a"],["b"]] therefore you need to test what is inside the list of a list.