[FIX] sale: do not schedule activity when no `user_id`
[FIX] sale: do not schedule activity when no `user_id`

- The `_write` override on `sale.order` create a Next Activity for the
  SO's salesperson when changing the SO `invoice_status` to upselling.

  If the SO doesn't have a salesperson this code crashes since `user_id`
  is required on Next Activities.
  To avoid this crash we create the Next Activity only if a salesperson
  is set on the SO.

  OPW-1914087

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