production_lot_custom_attributes: search all attributes fails if no attributes are defined in the system

[Leonardo Pistone]

to reproduce:

on lastest 6.1 ocb install production_lot_custom_attributes, delete all attributes and in the search view of production lots, enter a string into "search all attributes". You get:

2014-01-31 16:27:44,195 25495 ERROR ? openerp.osv.osv: Uncaught exception
Traceback (most recent call last):
  File "/home/lpistone/work/oerp/server/openerp/osv/osv.py", line 129, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/lpistone/work/oerp/server/openerp/osv/osv.py", line 195, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/lpistone/work/oerp/server/openerp/osv/osv.py", line 183, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/lpistone/work/oerp/server/openerp/osv/orm.py", line 2251, in search
    return self._search(cr, user, args, offset=offset, limit=limit, order=order, context=context, count=count)
  File "/home/lpistone/work/oerp/server/openerp/osv/orm.py", line 4592, in _search
    query = self._where_calc(cr, user, args, context=context)
  File "/home/lpistone/work/oerp/server/openerp/osv/orm.py", line 4443, in _where_calc
    where_clause, where_params = e.to_sql()
  File "/home/lpistone/work/oerp/server/openerp/osv/expression.py", line 795, in to_sql
    q2 = stack.pop()
IndexError: pop from empty list