Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

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

@@ -1,6 +1,8 @@
 package com.thealgorithms.strings;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Test;