fix wrong email check condition in prev commit (v-add-letsencrypt-domain)

bin/v-add-letsencrypt-domain

@@ -71,7 +71,7 @@ if [ ! -z "$mail" ]; then
     is_boolean_format_valid "$mail" 'mail'
 fi
 
-if [ "$mail" = 'no' ]; then
+if [ -z "$mail" ] || [ "$mail" = 'no' ]; then
     mail=''
     is_system_enabled "$WEB_SYSTEM" 'WEB_SYSTEM'
     is_object_valid 'web' 'DOMAIN' "$domain"