ATLAS-5381: Handle empty/corrupted reference vertices during entity delete and purge traversal (#740) * ATLAS-5381: Handle empty/corrupted reference vertices during entity delete and purge traversal * ATLAS-5381: Add empty reference vertex delete-path tests in DeleteHandlerV1Test to cover addUpstreamProcessEntities, getColumnLineageEntities, and deleteEdgeReference --------- Co-authored-by: Sheetal Shah <sheetal.shah@cloudera.com>