jwt_authn: fix a bug where JWT with wrong issuer is allowed in allow_missing case (#15194)

[jwt] When allow_missing is used inside RequiresAny, the requests with JWT with wrong issuer are accepted. This is a bug, allow_missing should only allow requests without any JWT. This change fixed the above issue by preserving JwtUnknownIssuer in allow_missing case.

Signed-off-by: Wayne Zhang <qiwzhang@google.com>