[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.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)