FIX helpdesk_mgmt when disabling "Required Category" in portal settings

 - Disable "Required Category" in helpdesk settings
 - Create a ticket by /new/ticket with empty category

Get

File "/mnt/data/odoo-addons-dir/helpdesk_mgmt/controllers/main.py", line 73, in _prepare_submit_ticket_vals
    int(kw.get("category"))
ValueError: invalid literal for int() with base 10: ''

helpdesk_mgmt/controllers/main.py

@@ -70,7 +70,7 @@ def create_new_ticket(self, **kw):
 
     def _prepare_submit_ticket_vals(self, **kw):
         category = http.request.env["helpdesk.ticket.category"].browse(
-            int(kw.get("category"))
+            int(kw.get("category") or 0)
         )
         company = category.company_id or http.request.env.company
         vals = {