libsecureboot: do further checks on files without manifests

verify_prep can return VE_FINGERPRINT_NONE. Consider such scenario so
the VE_GEUSS heuristics works with files that likely will not have
fingerprints in the manifest file.

Obtained from: Hewlett Packard Enterprise
Reviewed by: sjg
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53940