Refactor null key test to use variable

Author: lmj798

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/GenericHashMapUsingArrayTest.java

@@ -97,7 +97,8 @@ void testContainsKey() {
     @Test
     void shouldThrowNullPointerExceptionForNullKey() {
         GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
-        Assertions.assertThrows(NullPointerException.class, () -> map.put(null, "value"));
+        String nullKey = null; // Use variable to avoid static analysis false positive
+        Assertions.assertThrows(NullPointerException.class, () -> map.put(nullKey, "value"));
     }
 
     @Test