Tasks have a default identity created during canonicalization. If this default
identity is somehow missing, we'll hit panics when trying to create and sign the
claims in the plan applier. Fallback to the default identity if it's missing
from the task.
This changeset will need a different implementation in 1.7.x+ent backports, as
the constructor for identities was refactored significantly in #23708. The panic
cannot occur in 1.6.x+ent.
Fixes: https://github.com/hashicorp/nomad/issues/23758
