[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.
config['rql-cache-size'] = config['rql-cache-size'] * 10
add_entity_type('CWDataImport')
from cubicweb.schema import CONSTRAINTS, guess_rrqlexpr_mainvars
for rqlcstr in rql('Any X,XT,XV WHERE X is CWConstraint, X cstrtype XT, X value XV,'
'X cstrtype XT, XT name IN ("RQLUniqueConstraint","RQLConstraint","RQLVocabularyConstraint"),'
'NOT X value ~= ";%"').entities():
expression = rqlcstr.value
mainvars = guess_rrqlexpr_mainvars(expression)
yamscstr = CONSTRAINTS[rqlcstr.type](expression, mainvars)
rqlcstr.cw_set(value=yamscstr.serialize())
print 'updated', rqlcstr.type, rqlcstr.value.strip()