efivar: Allow NULL paths in build_dp

Allow there to be NULL paths in buildp. This lets us return the device
path to the partition, as well as to files on the partition.

Sponsored by: Netflix
Reviewed by: corvink, manu, asomers
Differential Revision: https://reviews.freebsd.org/D38615

(cherry picked from commit 49fd6affdbfe6af53f119ebb27ff5e5e292e1f67)