@Test
    public void testPrettyPrintEmptyTree() {
        final Tree<String> tree = new Tree<>();
        final String expected = "";
        assertEquals(expected, tree.prettyPrint());
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/TreeTest.java