[FIX] account: tax_amount can be null in database

The `tax_amount` of move lines is by default set to `0.0`.
Nevertheless, this default value is set by Odoo,
not by postgresql.

This is therefore likely that the `tax_amount` is set as
null instead of 0.0, in database.

Therefore, when getting this value directly with a SQL
request, this is possible that `null` will be returned.

Therefore, in this specific case, `res.get(record.id, 0.0`
could return `False`, if the sum of `tax_amount` is `null`,
and try to multiply a boolean with an integer is not possible:
`_rec_get(rec) * rec.sign`

opw-633903

addons/account/account.py

@@ -1755,7 +1755,7 @@ class account_tax_code(osv.osv):
         res2 = {}
         for record in self.browse(cr, uid, ids, context=context):
             def _rec_get(record):
-                amount = res.get(record.id, 0.0)
+                amount = res.get(record.id) or 0.0
                 for rec in record.child_ids:
                     amount += _rec_get(rec) * rec.sign
                 return amount