[DS-2034] Check for null (unset) properties and replace with an indicator
Merge pull request #622 from mwoodiupui/DS-2034

[DS-2034] Problem in [Control Panel]->[Dspace Configuration] - org.dspace.app.xmlui.wing.WingInvalidArgument: The 'characters' parameter is required for list items.
DS-2120: IdentifierServiceImpl should log less aggressive

The IdentifierProvider API uses exceptions to state that an Identifier
for a specific DSO cannot be found. These exceptions shoudln't get
logged as error.
Merge pull request #623 from tuub/DS-2120

DS-2120: IdentifierServiceImpl should log less aggressive