Fixes for create-book

[jderuiter]

This PR addresses a couple of smaller issues when using create-book:

• When manually adding a new edition and you do not add a new author, you're asked whether you want to add the author ['']
• When creating a new book manually and leaving the author blank you're asked whether you want to create the author []
• When adding a new book and possible matches are found these are not displayed as data["book"] is always set for the subjects (even when none are provided) and when a possible match is selected it is not actually set as parent_work as parent_work was sent twice in the POST request (once the selected value and once an empty hidden field)

[mouse-reeve]

Thank you so much! Looks good