style: fix style issues

Author: alxkm

src/test/java/com/thealgorithms/datastructures/stacks/NodeStackTest.java

@@ -2,13 +2,13 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
-import static org.junit.jupiter.api.Assertions.assertNull;
 
-import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
 
 class NodeStackTest {
 
@@ -249,4 +249,4 @@ void testToString() {
         assertTrue(stackString != null, "toString should not return null");
         assertTrue(stackString.length() > 0, "toString should return non-empty string");
     }
-}
+}