Investigate nans in question_items

[jannikrdev]

No description provided.

[lisallreiber]

the nan in question items come from "Text Display" questions which can be filtered out for the analysis. They are only informative Texts for the survey, but there are no responses associated with them

now the question is, where should the filtering be happening?

[jannikrdev]

I implemented the filtering in the answers transformation part for now. If we need it later, it's just two lines of code that must be changed