fix: [S/MIME] don't sign e-mails if no signing key is set
fix: [S/MIME] don't sign e-mails if no signing key is set

- fixes e-mails not going out on instances where no signing key was provided

SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP