DATAJPA-280 - Added null guards to AuditingHandler access.

In case the AuditingEntityListener is declared globally and <jpa:auditing /> is not activated the listener is considered by the JPA infrastructure but not configured correctly. In particular no AuditingHandler gets injected which leads to NullPointerExceptions every time an entity is persisted. Added checks for null to the access and skip it if no AuditingHandler is present.