fix: ignore item-less maintenance visit for sr no

[ankush]

When the item code isn't present maintenance visit fails with this exception. It's technically harmless, but annoying.

---

App Versions

{
	"erpnext": "14.0.0-dev",
	"frappe": "14.0.0-dev"
}

Route

Form/Maintenance Visit/MAT-MVS-2022-00002

Trackeback

Traceback (most recent call last):
  File "apps/frappe/frappe/app.py", line 66, in application
    response = frappe.api.handle()
  File "apps/frappe/frappe/api.py", line 54, in handle
    return frappe.handler.handle()
  File "apps/frappe/frappe/handler.py", line 31, in handle
    data = execute_cmd(cmd)
  File "apps/frappe/frappe/handler.py", line 68, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "apps/frappe/frappe/__init__.py", line 1259, in call
    return fn(*args, **newargs)
TypeError: get_serial_nos_from_schedule() missing 1 required positional argument: 'item_code'

Request Data

{
	"type": "POST",
	"args": {},
	"headers": {},
	"error_handlers": {},
	"url": "/api/method/erpnext.maintenance.doctype.maintenance_schedule.maintenance_schedule.get_serial_nos_from_schedule"
}

Response Data

{
	"exception": "TypeError: get_serial_nos_from_schedule() missing 1 required positional argument: 'item_code'"
}