Required for ENG-8212.
In case of backup restore, the singleton device list will also have lastPrimarySignature, which wasn't accepted before for singleton device lists.

Existing tests still pass.
Tested manually with lastPrimarySignature being present:

• Passes with no last primary signature provided and none expected (previous behavior unchanged)
• Fails if signature provided but none expected
• Fails no signature is provided but one is expected
• Passes if valid signature is provided and it's expected (previous signing key provided)
• Fails if signature verification fails for given signing public key