Fix crash in egamma topocluster decorator

Fix very rare fix seen when running over data reported and discussed here

https://its.cern.ch/jira/browse/ATLEGAMDPD-140?focusedCommentId=3693400&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-3693400

When the cluster is missing, decorate the egamma object with some default values rather than failing

Tagging @christos and @turra