Fixes implicit dropping of unseen target levels in SMOTE-like PipeOps

[advieser]

closes #881

[advieser]

Concerning the empty target levels, we decided to drop them in PipeOpSmoteNC from the themis package as well to have consistent behavior, i.e. dropping empty target levels instead throwing an error (which themis would do itself otherwise).
Side note: The other PipeOp from themis, PipeOpTomek, only down-samples. Because of this, there is no error for empty target levels in themis. Additionally, it does not modify the task internally, so no fix is needed.