fix 'undefined method' if relnotes_url is empty (bnc#877705)

also distinguish nil and "" in the message
veriosn + changelog
Merge remote-tracking branch 'yast/master' into pr/203

adjusting the version and changelog
Merge pull request #203 from yast/undefined-method-empty-relnotes-url

Fixed "undefined method" if an add-on has empty release notes URL