[CrOS PhoneHub] Do not access SerializedNavigationEntry vector if empty.

Accessing an empty sessions::SessionTab's |navigations| vector is
resulting in an out of bounds access, causing
BrowserTabsMetadataFetcherImpl to crash.

Fixed: 1142562
Bug: 1106937
Change-Id: I0ec890135f6acafd61a39a7d06b52ef1956bed96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522497


Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824826}