Fix wordcloud answer validation

The previous validation did not allow an empty List for texts which prevented abstentions. Furthermore, the validation of list elements was changed to require at least a single non-whitespace character. This is more correct but currently has no effect, because custom logic filters blank elements before validation.

While the validation has always been incorrect, it was not applied until recently.

Closes: #168 (closed)
Regression-MR: !533 (merged)