[16.0] [FIX] helpdesk_mgmt when disabling "Required Category" in portal settings

[LorenzoC0]

Same fix as #610

Tested in a 16.0 environment, the problem still occurs.

• 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: ''