[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.
Latest changes
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* table of `all latest changes`_
Links below is providing useful RQL query example.
.. _all latest changes: view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30