Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

src/main/java/com/thealgorithms/stacks/ReverseStringUsingStack.java

@@ -11,7 +11,7 @@ private ReverseStringUsingStack() {
      * @return reversed string
      */
     public static String reverse(String str) {
-        if(str==null) {
+        if (str == null) {
             throw new IllegalArgumentException("Input string cannot be null");
         }
         Stack<Character> stack = new Stack<>();

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

@@ -66,7 +66,7 @@ public static String reverse3(String string) {
      * and then pops them off to create the reversed string.
      */
     public static String reverse4(String str) {
-        if(str==null) {
+        if (str == null) {
             throw new IllegalArgumentException("Input string cannot be null");
         }
         Stack<Character> stack = new Stack<>();