[NavCapture] Foolproof return value

This CL is a small refactor to make it obvious when the function returns
`std::nullopt`.

Bug: none
Test: build for OS=chromeos
Change-Id: Id8e14fd869bd8ce2b08e920f910914764bf84819
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6576029
Auto-Submit: David Jacobo <djacobo@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: David Jacobo <djacobo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1463827}