Permit empty domain in transforming into enum.
Permit empty domain in transforming into enum.

toEnum call can be call on a vector which does not contains any domain
values, only NA. To preserve toEnum contract (every call returns a new
vector), we permit this situation.

111t8e/h2o-2,vbelakov/h2o,h2oai/h2o,h2oai/h2o,h2oai/h2o-2,vbelakov/h2o,rowhit/h2o-2,h2oai/h2o-2,h2oai/h2o-2,eg-zhang/h2o-2,100star/h2o,calvingit21/h2o-2,100star/h2o,111t8e/h2o-2,h2oai/h2o-2,rowhit/h2o-2,calvingit21/h2o-2,calvingit21/h2o-2,calvingit21/h2o-2,calvingit21/h2o-2,111t8e/h2o-2,vbelakov/h2o,elkingtonmcb/h2o-2,h2oai/h2o-2,100star/h2o,h2oai/h2o,111t8e/h2o-2,elkingtonmcb/h2o-2,100star/h2o,h2oai/h2o,rowhit/h2o-2,vbelakov/h2o,h2oai/h2o,elkingtonmcb/h2o-2,vbelakov/h2o,eg-zhang/h2o-2,eg-zhang/h2o-2,vbelakov/h2o,elkingtonmcb/h2o-2,111t8e/h2o-2,elkingtonmcb/h2o-2,h2oai/h2o,elkingtonmcb/h2o-2,100star/h2o,elkingtonmcb/h2o-2,eg-zhang/h2o-2,elkingtonmcb/h2o-2,h2oai/h2o-2,calvingit21/h2o-2,h2oai/h2o,elkingtonmcb/h2o-2,100star/h2o,rowhit/h2o-2,111t8e/h2o-2,rowhit/h2o-2,calvingit21/h2o-2,rowhit/h2o-2,eg-zhang/h2o-2,vbelakov/h2o,h2oai/h2o-2,elkingtonmcb/h2o-2,h2oai/h2o-2,rowhit/h2o-2,rowhit/h2o-2,111t8e/h2o-2,rowhit/h2o-2,h2oai/h2o,111t8e/h2o-2,vbelakov/h2o,111t8e/h2o-2,calvingit21/h2o-2,100star/h2o,h2oai/h2o,111t8e/h2o-2,h2oai/h2o-2,eg-zhang/h2o-2,rowhit/h2o-2,h2oai/h2o,100star/h2o,eg-zhang/h2o-2,eg-zhang/h2o-2,eg-zhang/h2o-2,calvingit21/h2o-2,100star/h2o,eg-zhang/h2o-2,calvingit21/h2o-2,h2oai/h2o-2,vbelakov/h2o,vbelakov/h2o