title: How to specify the output format for Null node in yaml-cpp?
body: <p>I use current(2013/12/12) latest revision of yaml-cpp.</p>
  
  <p>I noticed that both <code>YAML::Load("")</code>
  and <code>YAML::Load("---\n...")</code> results in a Null node,
  i.e. <code>YAML::Load("").Type() == YAML::NodeType::Null</code>
  yet when sent to <code>std::ostream</code> the former outputs an empty string
  but the latter outputs tilde (<code>~</code>).</p>
  
  <p>I want both nodes to output an empty string.
  How can I achieve it?</p>
  
  <p>I want to do so because I noticed tilde is interpreted as a string when I use <code>YAML::Load</code>.</p>
  
  <p>Here is the code to show this difference.</p>
  
  <pre><code>#include &lt;yaml-cpp/yaml.h&gt;
  #include &lt;iostream&gt;
  #include &lt;cassert&gt;
  
  int main() {
      YAML::Node node1 = YAML::Load("");
      YAML::Node node2 = YAML::Load("---\n...");
  
      assert (node1.Type() == YAML::NodeType::Null);
      assert (node2.Type() == YAML::NodeType::Null);
  
      std::cout &lt;&lt; node1 &lt;&lt; std::endl;
      std::cout &lt;&lt; node2 &lt;&lt; std::endl;
  }
  
  /* OUTPUT:
  
  ~
  */
  </code></pre>