Empty archive_unless_directed cause import errors

[FiloSottile]

A clause like

    filter {
      has [FOO]
    }.archive_unless_directed

will still generate two filters, one for directed emails and one for not, but the latter one will be empty, causing an import error.

[antifuchs]

Eeep, yes, that's a messy edge case. I think it would make sense in archive_unless_directed to check if the new filter is empty / identical to the base filter and not return a new one in that case (I don't have all the state swapped in at the moment, it might need some refactoring to let us not add a filter from this).