Prevented NullPointerException when closing a null session

Since the end-state of closeSession() is to have a null session, if
session is already null, there is nothing to do.

src/main/java/edu/unl/cse/csce361/hibernate/FruitPopulator.java

@@ -28,6 +28,7 @@ public class FruitPopulator {
     }
 
     public static void closeSession(Session session) {
+        if (session != null) {
             try {
                 session.close();
             } catch (HibernateException hibernateException) {
@@ -38,6 +39,7 @@ public class FruitPopulator {
                 session = null;
             }
         }
+    }
 
     private void makeFruitSalad() {
         Session session = getSession();