[FIX] account_voucher: generate empty context to avoid traceback

bzr revid: mat@openerp.com-20130821121714-5q5qa56suab0dehs

diff --cc addons/account_voucher/account_voucher.py
index 4f383a1,69e8405..ef2b861
--- 1/addons/account_voucher/account_voucher.py
--- 2/addons/account_voucher/account_voucher.py
+++ b/addons/account_voucher/account_voucher.py
@@@ -893,6 -893,8 +893,8 @@@ class account_voucher(osv.osv)
          return res
  
      def onchange_journal(self, cr, uid, ids, journal_id, line_ids, tax_id, partner_id, date, amount, ttype, company_id, context=None):
+         if context is None:
 -            context ={}
++            context = {}
          if not journal_id:
              return False
          journal_pool = self.pool.get('account.journal')