PYSEC-2026-2528 joserfc: HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)

Summary joserfc.jwt.decode accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or None. HMACAlgorithm.sign and HMACAlgorithm.verify in src/joserfc/rfc7518/jwsalgs.py:62-70 feed whatever OctKey.getopkey... produced into hmac.new..., and...