bruschetta: Ensure we can actually install without a pflash file

This was always intended to work by the policy code, but in fact would
crash during install because the installer assumed the pflash file was
always set.

Bug: b:264495837
Change-Id: I4a52e94b4d8c3290f877f671d656d2efcff3d0e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4431961
Reviewed-by: Emil Mikulic <easy@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1132330}