Fix issue media search does not search child items

[NguyenThuyLan]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes

Description

This PR is related to issue #16968
I fixed this issue by calling GetPagedDescendantsAsync function if search filter has data, otherwise if search filter has no data, it will be filtered as before.

How can test these changes:

1. Go to the Media section, create a folder named "Product", create a new image named "tablet", inside the folder "Product", create a new image named "table"
2. On the Media section, search for "tab" => results will show both images of "table" and "tablet"
3. Go to the "Product" folder and search for "tab" => results will show an image of "table".