[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.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')