[FIX] account_tax_python: empty code

If the user somewhat removes the 'Applicable Code' text, the call to
`safe_eval` will crash.

opw-769267

addons/account_tax_python/account_tax_python.py

@@ -36,9 +36,9 @@ class AccountTaxPython(models.Model):
 
     @api.v8
     def compute_all(self, price_unit, currency=None, quantity=1.0, product=None, partner=None):
-        taxes = self.env['account.tax']
+        taxes = self.filtered(lambda r: r.amount_type != 'code')
         company = self.env.user.company_id
-        for tax in self:
+        for tax in self.filtered(lambda r: r.amount_type == 'code'):
             localdict = {'price_unit':price_unit, 'quantity': quantity, 'product':product, 'partner':partner, 'company': company}
             safe_eval(tax.python_applicable, localdict, mode="exec", nocopy=True)
             if localdict.get('result', False):