CVE-2026-49852: joserfc: HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)

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/jws_algs.py:62-70 feed whatever OctKey.get_op_key(...) produced into hmac.new(...), and OctKey.import_key only emits a SecurityWarning when the raw key is shorter than 14 bytes without rejecting zero-length input. Any application whose JWT secret is sourced from an unset environment variable, an unset Redis / DB row, a key finder fallback that returns "", or a Hash.new("")-style default verifies attacker tokens forged with HMAC(key=b"", signing_input) because the attacker trivially reproduces the same digest with no secret knowledge.

This is a cross-language sibling of jwt/ruby-jwt GHSA-c32j-vqhx-rx3x / CVE-2026-45363 (HS256/HS384/HS512 verify accepted an empty/nil HMAC key, filed 2026-05-13). ruby-jwt v3.2.0 added an ensure_valid_key! precondition that rejects empty keys at both sign and verify entry; joserfc has no equivalent. (The same primitive lives in the deprecated authlib.jose module by the same maintainer; filing this advisory against joserfc alongside a separate authlib advisory because the codebases are independent shipping artifacts on PyPI.)