make blitems.guid nullable (bug 1042384)

fixes [bug 1042384](https://bugzilla.mozilla.org/show_bug.cgi?id=1042384)

For context:
- the field was made nullable in the code with https://github.com/mozilla/olympia/commit/b334b2f710b49986ff58ce5ce18f1f644d85825c
- the field wasn't nullable in the database, according to https://bugzilla.mozilla.org/show_bug.cgi?id=560265#c1