Refactored ReverseStringUsingStack

Author: navaneedan07

src/test/java/com/thealgorithms/strings/ReverseStringTest.java

@@ -37,4 +37,14 @@ public void testReverseString3(String input, String expectedOutput) {
     public void testReverseString4(String input, String expectedOutput) {
         assertEquals(expectedOutput, ReverseString.reverse4(input));
     }
+
+    @ParameterizedTest
+    @MethodSource("testCases")
+    public void testReverseString4WithNullInput(String input, String expectedOutput) {
+        if (input == null) {
+            assertThrows(IllegalArgumentException.class, () -> ReverseString.reverse4(input));
+        } else {
+            assertEquals(expectedOutput, ReverseString.reverse4(input));
+        }
+    }
 }