[FIX] account: avoid computing uom price if no product

Before this commit:

* Enable UOMs
* Create a new invoice and invoice line
* Set *only* the uom field of the invoice line
* Singleton expected error

This happens because the method _compute_price of `uom.uom` expects a
singleton, however since no product has been defined yet, it gets an
empty recordset which of course doesn't comply with self.ensure_one(),
raising a Singleton Error.

Functionally speaking, it makes sense to define a product before
choosing an uom, however a Singleton Error is an unexpected error, so
instead of doing that, we only calculate the price_unit if a product has
been chosen.

Fixes #28269

closes odoo/odoo#28278

addons/account/models/account_invoice.py

@@ -1756,14 +1756,14 @@ class AccountInvoiceLine(models.Model):
         result = {}
         if not self.uom_id:
             self.price_unit = 0.0
-        else:
+
+        if self.product_id and self.uom_id:
             if self.invoice_id.type in ('in_invoice', 'in_refund'):
                 price_unit = self.product_id.standard_price
             else:
                 price_unit = self.product_id.lst_price
             self.price_unit = self.product_id.uom_id._compute_price(price_unit, self.uom_id)
 
-        if self.product_id and self.uom_id:
             if self.product_id.uom_id.category_id.id != self.uom_id.category_id.id:
                 warning = {
                     'title': _('Warning!'),