mgmt/osdp: phy: Allow non-conformant, 0 length, encrypted data blocks

If command or reply has no data, PD "should" use secure message types
SCS_15 or SCS_16. But some PD seem to not implement this correctly. We
will be tolerant towards those faulty implementations.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>