Fix blank messages : store as a draft if it is invalid

ui/compose.lua

@@ -45,6 +45,23 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
         if msg_id[name] then
             id = msg_id[name]
         end
+        if (fields.to == "" and fields.cc == "" and fields.bcc == "") or fields.body == "" then -- if mail is invalid then store it as a draft
+            local id = mail.new_uuid()
+            if msg_id[name] then
+                id = msg_id[name]
+            end
+            mail.save_draft({
+                id = id,
+                from = name,
+                to = fields.to,
+                cc = fields.cc,
+                bcc = fields.bcc,
+                subject = fields.subject,
+                body = fields.body
+            })
+            mail.show_mail_menu(name)
+            return
+        end
         local success, err = mail.send({
             id = id,
             from = name,