[7.0] moves can have tax amount NULL -> _sum in account.tax.code crashes

[Leonardo Pistone]

Observed in v7.0.

The field tax_amount of the move_line can be NULL (as opposed to 0.0).

If you have all moves with a certain tax code with a tax_amount = NULL, and then open the form view of the tax code from the web interface, it crashes because it is trying to round a NULL.

  File "/home/lpistone/w/oe/server/openerp/osv/osv.py", line 132, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/lpistone/w/oe/server/openerp/osv/osv.py", line 199, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/lpistone/w/oe/server/openerp/osv/osv.py", line 187, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/lpistone/w/oe/server/openerp/osv/orm.py", line 3622, in read
    result = self._read_flat(cr, user, select, fields, context, load)
  File "/home/lpistone/w/oe/server/openerp/osv/orm.py", line 3743, in _read_flat
    res2 = self._columns[f].get(cr, self, ids, f, user, context=context, values=res)
  File "/home/lpistone/w/oe/server/openerp/osv/fields.py", line 1142, in get
    result = self._fnct(obj, cr, uid, ids, name, self._arg, context)
  File "/home/lpistone/w/oe/addons/account/account.py", line 1852, in _sum_year
    where=where, where_params=where_params)
  File "/home/lpistone/w/oe/addons/account/account.py", line 1829, in _sum
    res2[record.id] = round(_rec_get(record), obj_precision.precision_get(cr, uid, 'Account'))
TypeError: a float is required

Thanks!