Fix rare crash in egamma topocluster decorator
Fix rare crash when cluster associated egamma particle is missing, see https://its.cern.ch/jira/browse/ATLEGAMDPD-140?focusedCommentId=3693400&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-3693400
Rather than failing, decorate egamma object with some default values