oai_dc should output keywords

Created by: yytosi

There is a condition in oai_dc.genshi which mandates that a keyword must have value. This condition is faulty and should be removed.

https://bitbucket.org/tietoarkisto/kuha_oai_pmh_repo_handler/src/1.0.0/kuha_oai_pmh_repo_handler/templates/oai_dc.xml#lines-26