Update quiz handling

[cansavvy]

Purpose/implementation Section

What changes are being implemented in this Pull Request?

Originally, I had set this up so you could say quiz_dir = NULL and the quiz parts would be skipped, but there was one part that was missing that handling.

[cansavvy]

I tested this locally on OTTR_Template files using:

Rscript -e "ottrpal::bookdown_to_embed_leanpub(
            render = FALSE, \
            chapt_img_key = 'resources/chapt_screen_images/chapter_urls.tsv', \
            make_book_txt = TRUE, \
            quiz_dir = NULL)"

and it worked!