Do not tag <edition> if user leaves field blank in UI
cc @jordandc
Expected behaviour
Metadata fields left blank by user in the UI should not be written in chapter-processed book metadata so that book metadata display accurately on NCBI Bookshelf site.
Current behaviour
When the book edition field is left blank by a user, the BCMS writes dummy text it provides into an element causing display issues.
Steps to reproduce
<edition>null</edition>
Priority
Y, for deployment