Remove unused css: Prevent failure when book has invalid items marked as stylesheets
Check Book: Add checks for empty HTML/XML/CSS/image files in the book.  Fixes #1329971 [Private bug](https://bugs.launchpad.net/calibre/+bug/1329971)
Fix #1329777 [Translation DE](https://bugs.launchpad.net/calibre/+bug/1329777)
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Change NON NULL to NOT NULL in the sqlite schema
Change the test db to also use the NOT NULL constraint form
oops