Document info box shows publication date of unpublished document

The org.hippoecm.frontend.editor.workflow.dialog.DocumentMetadataDialog is not adapted to the new way of storing document variants in 7.9. The publication date of the node with hippostd:state = published is always shown when that node is found. Good for 7.8, but in 7.9 that node might be there, if the document once reached a published state.