[bookmarks] Skip null children when fetching images for folders

It's possible for children to be removed while folder images are being
fetched. It's possible to display folders and their children
side-by-side in the root bookmarks screen. When this happens and one
of the children being queried returns null, the image fetcher crashes.
Adding code to check for null when querying folder children and
skipping them.

Bug: 1446932
Change-Id: I2aeb8e93274e2f02c215fd24047adfa216e67eff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4552418


Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1147338}