SHAKE: ACVP server doesn't generate empty messages for registrations explicitly declared capability via 'inEmpty' tag

The [SHA3 and SHAKE Algorithm Capabilities Registration](https://pages.nist.gov/ACVP/draft-celi-acvp-sha3.html#name-sha3-and-shake-algorithm-ca) allows declaring null (zero-length) messages support via the 'inEmpty' tag. However, the test vector set doesn't generate cases for null messages.
 