allow empty selections for some steps

[topepo]

If step_other, step_dummy, step_corr, step_linearcomb and a few others have selectors that do not return any columns, go ahead and return the data as-is.

[AnselmJeong]

Yes, step_dummy allows empty selection, so you can define like step_dummy(all_nominal()) with a data.frame containing no factor variables. But it did not help anyway, because it still fails at the prep stage.

[topepo]

@AnselmJeong please test; I just committed a PR that may have solved this for you.