Add unit tests for ReverseStringUsingStack

Author: navaneedan07

src/test/java/com/thealgorithms/stacks/ReverseStringUsingStackTest.java

@@ -35,4 +35,11 @@ void testSingleCharacter() {
     void testWhitespaceHandling() {
         assertEquals("dlroW olleH", ReverseStringUsingStack.reverse("Hello World"));
     }
-}
+
+    @Test
+    void testNullInput() {
+        assertThrows(NullPointerException.class, () -> {
+            ReverseStringUsingStack.reverse(null);
+        });
+    }
+}