Fix: Wiki_dpr - fix when with_embeddings is False or index_name is "no_index"
Fix the bugs noticed in #1915 

There was a bug when `with_embeddings=False` where the configuration name was the same as if `with_embeddings=True`, which led the dataset builder to do bad verifications (for example it used to expect to download the embeddings for `with_embeddings=False`).

Another issue was that setting `index_name="no_index"` didn't set `with_index` to False.

I fixed both of them and added dummy data for those configurations for testing.