Clixon backend transaction callback fails for empty types

[olofhagsand]

Given YANG:

    choice c {
        leaf first { type empty; }
        leaf second { type empty; }
    }

An edit in the datastore such as: <first/> -> <second/> will not be visible in the transaction (see https://clixon-docs.readthedocs.io/en/latest/backend.html#transactions).