[FIX] account_tax_python: empty code
[FIX] account_tax_python: empty code

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

opw-769267

ygol/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo