libsecureboot: Do not propagate empty string

If Trust Anchors are provided by UEFI and not compiled into
libsecureboot the segmentation fault occurs due to empty
or NULL string usage.

Obtained from: Semihalf
Reviewed by: sjg
Differential revision: https://reviews.freebsd.org/D35120

(cherry picked from commit e6ef5042e485f74e7233a9974010b16a7316167e)