Use ndata instead of len

Created by: scarlehoff

Fixes #1674 (closed)

I'll think of a better way of testing https://github.com/NNPDF/nnpdf/issues/1674#issuecomment-1438335958 this afternoon I don't like checking the string itself but of course needed to check the str so ¯_(ツ)_/¯

But if you want to have the fix merged asap, @Zaharid, feel free to merge this.