Fix DevVoid decorated commands

Closes #733 (closed)

We used as default value for command type None. So when we get None we start to look for type hints for command's method. But None can also mean, that user wants to force command to be DevVoid.

Now we use as default option internal no-opt class _DevVoid. The small disadvantage is that e.g. PyCharm now shows this value as default one (insted of None of DevVoid) - but I think this is not a big problem