[FIX] account: methods calling reverse_moves expects a list

The methods calling `reverse_moves` expects a list
as returned result.
For instance, in `account_move.py`, `account.partial.reconcile`,
method unlink:
```
reversed_moves = exchange_rate_entries.reverse_moves()
for rev_move in self.env['account.move'].browse(reversed_moves):
```

If there was no reversed moves, the method returned a boolean,
and a list otherwise. It should always return a list, empty
in the above first case.

opw-669790

addons/account/models/account_move.py

@@ -211,7 +211,7 @@ class AccountMove(models.Model):
             reversed_moves._post_validate()
             reversed_moves.post()
             return [x.id for x in reversed_moves]
-        return True
+        return []
 
     @api.multi
     def open_reconcile_view(self):