signlk: remove unset fields

Some fields in the certificate's DN might not be set (or might be empty). Newer
version of openssl (e.g. 1.1+) have a stricter DN parsing and fail to parse
empty (or too short) fields. For example, we can see such errors:

140708458570880:error:0D07A098:asn1 encoding routines:ASN1_mbstring_ncopy:string
too short:crypto/asn1/a_mbstr.c:102:minsize=1

Removing the empty (or short) fields fixes the openssl error, and XBL is still
able to authenticate the ELF file.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>