Fix: stop isbns from being <Nothing> in view.html

[scottbarnes]

For some reason edition.to_isbn10() and edition.to_isbn13 seem to be returning the good for nothing <Nothing> type in view.html.

This commit opts for None over other falsy values when setting isbn_10 and isbn_13.

Fix

Testing

Try to load a work or edition:

• http://localhost:8080/books/OL20431791M
• https://staging.openlibrary.org/works/OL118388W/Flatland?edition=key%3A/books/OL37044775M.

[jimchamp]

LGTM --- thanks, @scottbarnes!

[jimchamp]

@scottbarnes, I just remembered that we have a NothingEncoder that may help out in cases similar to this.
The encoder is defined here, and there's a usage example here.

This probably wouldn't have worked in this case. If I recall correctly, the dict was encoded in a template using json_encode, not by a json.dump call.