[FIX] base: allow removing the contact barcode

Steps to reproduce the bug:
- Go to any Contact > Sales and Purchase tab
- Edit > Change the barcode, and click Save
- Then click Edit, and try to erase (empty) the barcode > save

Problem:
A validation error is triggered, the constraint will check if other partners do not have the same barcode
but as it is empty there will be several other partners

Solution:
When the barcode is empty, no need to check if other partners have the same

opw-2753291

closes odoo/odoo#84411

Signed-off-by: William Henrotin (whe) <whe@odoo.com>

odoo/addons/base/models/res_partner.py

@@ -398,7 +398,7 @@ class Partner(models.Model):
 
     @api.constrains('barcode')
     def _check_barcode_unicity(self):
-        if self.env['res.partner'].search_count([('barcode', '=', self.barcode)]) > 1:
+        if self.barcode and self.env['res.partner'].search_count([('barcode', '=', self.barcode)]) > 1:
             raise ValidationError('An other user already has this barcode')
 
     def _update_fields_values(self, fields):