Add a check of the existance of a next node (#3051)

* Fix #2976

Co-authored-by: Sahil Prafulkumar Parekh <sh883193@dal.ca>
Co-authored-by: Yang Libin <contact@yanglibin.info>

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/HashMap.java

@@ -89,7 +89,9 @@ public class HashMap {
         public void delete(int key) {
             if (!isEmpty()) {
                 if (first.getKey() == key) {
-                    first = null;
+                    Node next = first.next;
+                    first.next = null; // help GC
+                    first = next;
                 } else {
                     delete(first, key);
                 }