focus-mode: Fix crash to focus on chip_carousel with empty string

This CL will resolve the crash that when unfocus the textfield with an
empty string and to focus on the chip_carousel, whose `SetTasks` will
remove all children first and then add new children based on tasks.

We will call this function only when `chip_carousel` is invisible.

Bug: b/315179613
Test: Manual
Change-Id: Ia8979d86ab97bc3686ce33237f4513b59988bc5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5096608
Reviewed-by: Richard Chui <richui@chromium.org>
Commit-Queue: Hongyu Long <hongyulong@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1234313}