by default this is not the widget responsability to turn empty string into None,
move this behaviour to the field.
include README
include pylintrc
include bin/cubicweb-*
include man/cubicweb-ctl.1
recursive-include doc *.txt *.zargo *.png *.html makefile *.rst
recursive-include misc *
recursive-include web/data *
recursive-include web/wdoc *.rst *.png *.xml ChangeLog*
include web/views/*.pt
recursive-include etwist *.xml *.html
recursive-include i18n *.pot *.po
recursive-include schemas *.py *.sql.*
recursive-include entities/test/data *
recursive-include sobjects/test/data *
recursive-include server/test/data *
recursive-include server/test sources*
recursive-include web/test/data *.js *.css *.png *.gif *.jpg *.ico external_resources
recursive-include devtools/test/data *
recursive-include skeleton *.py *.css *.js *.po compat *.in *.tmpl
prune misc/cwfs