PyJWT: HS256/384/512 verify accepts empty HMAC key via oct JWK (PyJWK/PyJWKClient) enabling token forgery

Summary HMACAlgorithm.fromjwk accepts a symmetric kty=oct JWK whose k value is the empty string and returns a zero-length key without raising. When a token is then verified with that key object a PyJWK, or a PyJWK resolved by PyJWKSet / PyJWKClient, the HMAC verifier computes HMACkey=b"",...