cert: replace cert->getIssuerUID() by cert->issuer->getId()

getIssuerUID() return the DN of the cert, but can be empty. Prefer
to use the public key instead.

Change-Id: I02e1c7d9d8d388c805cf129b0e42064ffbd0ee5b
GitLab: https://git.jami.net/savoirfairelinux/ring-project/-/issues/1282