Nullability fine-tuning based on IntelliJ IDEA 2018.3 inspection

Issue: SPR-15540

spring-orm/src/main/java/org/springframework/orm/hibernate5/SessionHolder.java

@@ -50,6 +50,7 @@ public class SessionHolder extends EntityManagerHolder {
 
 
 	public SessionHolder(Session session) {
+		// Check below is always true against Hibernate >= 5.2 but not against 5.0/5.1 at runtime
 		super(EntityManager.class.isInstance(session) ? session : null);
 		this.session = session;
 	}