rename primary key sequence only if it exists

cc/ @sgrif @rafaelfranca @senny

When a table has  no sequence in the  for primary key (for example :uuid), it was not possible to rename it without using sql. This fix it.

I think it's still worth having the assertion that the table was renamed here, in addition to the `assert_nothing_raised`, so we know work was still done, and not something like `rollback_transaction unless seq`