chore: add tests testZigZagCaseWithNullChild()

Author: sozelfist

src/test/java/com/thealgorithms/datastructures/trees/SplayTreeTest.java

@@ -8,6 +8,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 
@@ -110,4 +111,17 @@ private SplayTree createComplexTree() {
 
         return tree;
     }
+
+    @Test
+    public void testZigZagCaseWithNullChild() {
+        SplayTree tree = new SplayTree();
+        tree.insert(30);
+        tree.insert(10);
+        tree.insert(20);
+        tree.insert(15);
+
+        assertTrue(tree.search(15));
+
+        assertEquals(Arrays.asList(10, 15, 20, 30), tree.traverse(SplayTree.IN_ORDER));
+    }
 }