[FIX] account: Passed context as dict if it is None

lp bug: https://launchpad.net/bugs/691810 fixed

bzr revid: pso@tinyerp.com-20101220055129-5anvbd7pr09x0q1w

addons/account/account.py

@@ -1278,6 +1278,8 @@ class account_move(osv.osv):
         return super(account_move, self).copy(cr, uid, id, default, context)
 
     def unlink(self, cr, uid, ids, context=None, check=True):
+        if context is None:
+            context = {}
         toremove = []
         obj_move_line = self.pool.get('account.move.line')
         for move in self.browse(cr, uid, ids, context=context):