Don't use std::optional in ModelsFetchedCallback

This change simplifies the type used for passing the fetched model
response. A nullptr now indicates the absence of a response, removing
the need for the std::optional wrapper.

Change-Id: Ib1c4ce212f7b0240f7e85cd71989614e2ef33492
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6854821
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502214}