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.

Let's have `pk_and_sequence_for` return a `NullSequence` that has an `identifier` of `nil`. This way the calling could should not need to change at all.