[omnibox][ml] Fix crash when running linear-mapped scoring logic.

When executing the linear-mapped scoring variant in client code, it's
possible for the AutocompleteController to invoke the ML scoring service
before the scoring model has been fully loaded into memory. Doing so
will result in the model returning std::nullopt as the prediction value
associated with all URL suggestions.

Since we didn't originally account for the presence of std::nullopt in
the original code, this results in the client crashing when encountering
this scenario. As such, this CL fixes the issue by defaulting to a
prediction value of zero when the model returns std::nullopt.

Bug: 330695208,40062540
Change-Id: I8b1834c4b800f21604577e9d857a4f53472f5fc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398677
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282810}