Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

src/main/java/com/thealgorithms/strings/ReverseString.java

@@ -66,6 +66,7 @@ public static String reverse3(String string) {
      * and then pops them off to create the reversed string.
      */
     public static String reverse4(String str) {
+        // Check if the input string is null
         if (str == null) {
             throw new IllegalArgumentException("Input string cannot be null");
         }
@@ -85,4 +86,4 @@ public static String reverse4(String str) {
         }
         return reversedString.toString();
     }
-}
+}