Safeguard against missing ct_config.pb file

This change adds a defense in depth fix to handle cases where
ct_config.pb may not be on disk, it treats that case as similar to having
invalid configuration file.

Change-Id: I20dc4e9f03a10f9c96a77d90de5dc03398779013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5227068
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sushanth Rajasankar <Sushraja@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1250944}