[views] csvexport accept an empty rset (closes #4236928) When you tried to apply the 'csvexport' view on an empty rset, the view couldn't be selected and you got a HTTP 500 error. Also add two new test cases.

include *.py
include */*.py
recursive-include data *.gif *.png *.ico *.css *.js
recursive-include i18n *.po
recursive-include wdoc *