[ATLPHYSVAL-771] Fix unchecked element links in PFlow physics validation
This MR adds validity checks to some element links in the PFlow physics validation code.
The changes are intended as a short-term fix to the blocking of https://its.cern.ch/jira/browse/ATLPHYSVAL-771 by a crash, to allow this validation step to proceed. They do not fix the underlying problem leading to the invalid links, and the warning messages that will now start appearing in these situations can hopefully help identify the underlying issue.