[server,bfss] fix storage migration when Bytes attributes is None (closes #13519541) When a Bytes attributes is None before the BFSS storage migration, the migration crashes. A Bytes attribute can be None, when the attribute is not required and there is no default value. In this case, the attributes should remained None after migration and no file is stored on the disk.

usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc