creds: rename "tpm2-absent" encryption to "null" encryption

creds: rename "tpm2-absent" encryption to "null" encryption

This is what it is after all: encryption with a NULL key. This is more
descriptive, but also relevant since we want to use this kind of
credentials in a different context soon: for carrying pcrlock data into
a UKI. In that case we don#t want encryption, since the pcrlock data is
intended to help unlocking secrets, hence should not be a secret itself.

This only changes the code labels and the way this is labelled in the
output. We retain compat with the old name.