Created an explicit zero-OID which can be used for Subject
Public Key that do not have their own key identifier.
With this, all keys (including the subject public key) have
a proper key OID string so we don't need to make a special
case of null pointers when it comes to handling key OIDs.
Change-Id: Ice6923951699b6e253d7fd87e4c1b912470e0391
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
