Fix to_s empty content

Render '<Empty>' when a node has nil content and add tests for both
nil and non-nil content values.

Author: evolve75

lib/tree.rb

@@ -318,7 +318,8 @@ def marshal_load(dumped_tree_array)
     #
     # @return [String] A string representation of the node.
     def to_s
-      "Node Name: #{@name} Content: #{@content.to_s || '<Empty>'} " \
+      content_str = @content.nil? ? '<Empty>' : @content.to_s
+      "Node Name: #{@name} Content: #{content_str} " \
         "Parent: #{root? ? '<None>' : @parent.name.to_s} "       \
         "Children: #{@children.length} Total Nodes: #{size}"
     end

test/test_tree.rb

@@ -94,6 +94,7 @@ def test_root_setup
       assert_not_nil(@root.name, 'Name should not be nil')
       assert_equal('ROOT', @root.name, "Name should be 'ROOT'")
       assert_equal('Root Node', @root.content, "Content should be 'Root Node'")
+      assert(@root.to_s.include?('Content: Root Node'), 'to_s should include content value')
       assert(@root.root?, 'Should identify as root')
       assert(!@root.children?, 'Cannot have any children')
       assert(@root.content?, 'This root should have content')
@@ -117,6 +118,11 @@ def test_root
       assert_equal(2, @root.node_height, "Root's height after adding the children should be 2")
     end
 
+    def test_to_s_empty_content
+      empty = Tree::TreeNode.new('EMPTY')
+      assert_match(/Content: <Empty>/, empty.to_s)
+    end
+
     def test_from_hash
       #      A
       #    / | \