[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.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile