title: pyqt signal emit with object instance or None
body: <p>I would like to have a pyqt signal, which can either emit with a python "object" subclass argument or None. For example, I can do this:</p>
  
  <pre><code>valueChanged = pyqtSignal([MyClass], ['QString'])
  </code></pre>
  
  <p>but not this:</p>
  
  <pre><code>valueChanged = pyqtSignal([MyClass], ['None'])
  TypeError: C++ type 'None' is not supported as a pyqtSignal() type argument type
  </code></pre>
  
  <p>or this:</p>
  
  <pre><code>valueChanged = pyqtSignal([MyClass], [])
  TypeError: signal valueChanged[MyObject] has 1 argument(s) but 0 provided
  </code></pre>
  
  <p>I also tried None without the quotes and the c++ equivalent "NULL". But neither seems to work. What is it I need to do to make this work?</p>