conversation: verify conversationId before cloning

This patch validates the conversationId before proceeding with
cloning. Otherwise, an empty conversationId could lead to
accidental removal of the entire conversations directory.

https://git.jami.net/savoirfairelinux/jami-client-qt/-/issues/1982
Change-Id: I428247498bb7533b12dadb8a479779e228e6dfb6