fix: [mattermost] on first launch, there are no ``config.json``.

mattermost/hooks/postgres_database-relation-joined

@@ -18,8 +18,10 @@ services:
       MM_USERNAME: $USER
 "
 
+if [ -e "$SERVICE_CONFIGSTORE/mattermost/config/config.json" ]; then
     sed -ri  's/(^\s+"DataSource": "postgres:[^:]+:)([^@]+)@/\1'"$PASSWORD"'@/' \
         "$SERVICE_CONFIGSTORE/mattermost/config/config.json" || exit 1
+fi
 
 [ "$control" == "$(relation-get control 2>/dev/null)" ] && exit 0