Add unit tests for ReverseStringUsingStack

Author: navaneedan07

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

@@ -37,9 +37,6 @@ void testWhitespaceHandling() {
     }
 
     @Test
-    void testNullInput() {
-        assertThrows(NullPointerException.class, () -> {
-            ReverseStringUsingStack.reverse(null);
-        });
+    void testNullInput() { assertThrows(NullPointerException.class, () -> { ReverseStringUsingStack.reverse(null); });
     }
 }