* cipher: fix panic on KeyUnwrap of too-short slice

* jwe: don't call KeyUnwrap on empty (encrypted) key

Also don't call `aead.decrypt` on an empty key.

* test: make asymmetric_test more precise

These two test cases were passing a nil recipient, and checking for "any error"
instead of a specific error, which meant that introducing a nil recipient check
in `decryptKey` caused the test to stop testing what it meant to test, but
continue passing. Now we check for a specific error.

* test: TestKeyUnwrapShort

* jwe: TestEmptyEncryptedKey

* test: add `shorten` and `empty` corruptors