Currently a traceback is occurring when the user tries to create an invoice payment with referrer in invoice.

<b>To reproduce this issue:</b>

1) Install partner_commission without demo
2) Create a commission plan from crm with a rule containing category and rate 
3) Create a product with the same category used in the above commission rule 
4) Create a contact with commission plan and partner level in partner assignment 
5) Create a confirmed customer invoice with line as above product and referrer as
     above contact and register a payment.

<b>Error:- </b>
```
AttributeError: 'bool' object has no attribute 'partner_id'
```

The above traceback is occurring because we are trying to get the partner_id value from `_get_sales_representative` method.
https://github.com/odoo/enterprise/blob/313593cf176271060878dda5ebc677fa44b519db/partner_commission/models/account_move.py#L124-L125

But the above method tries to get the user_id from the sale order. Here in this case, we don't have any SO, so we the above method will return False.

https://github.com/odoo/enterprise/blob/313593cf176271060878dda5ebc677fa44b519db/partner_commission/models/account_move.py#L26-L27

This leads to the above traceback when parter_id is retrieved from the method.

<b>Solution:-</b>

Take the value of partner_id from purchase. Since the partner_id value is getting from move.referrer_id in purchase.

https://github.com/odoo/enterprise/blob/313593cf176271060878dda5ebc677fa44b519db/partner_commission/models/account_move.py#L46-L50

opw-4676967