OWLearnerWidget: Don't crash when no features
OWLearnerWidget: Don't crash when no features

If we provide a data set without any features (as unpreprocessed Corspus) learners crashed with a red screen. This PR adds a check for this and shows an error message instead.

qPCR4vir/orange3,cheral/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3