releasetools: Handle the case of not having ro.vendor.build.fingerprint.

Commit d572632f added support in build
system to blacklist given vendor properties. When
ro.vendor.build.fingerprint gets blacklisted, the OTA generation script
can't rely on those properties to determine a change in vendor images.
This CL considers such a case as "vendor images must have changed"
between the two builds.

Bug: 113892939
Test: Generate an incremental package with builds not having
      ro.vendor.build.fingerprint.
Test: python -m unittest test_ota_from_target_files
Change-Id: I188de9c3cbeecf26132c92b9356e9d5fef75205e