text: Title: Bug (?) from #2041
  Body: This new pattern in ``PlotCurveItem.setPen``, ``PlotCurveItem.setBrush``, and others has caused a number of breaks for me:
  
  https://github.com/pyqtgraph/pyqtgraph/blob/4e350d03ac2007c47cbfcca9d3b77a6a2a638e5c/pyqtgraph/graphicsItems/PlotCurveItem.py#L373-L380
  
  I believe it would be safer as
  
  ```python
  if args and args[0] is None:
      self.opts['pen'] = None
  ```
  
  As for now, I've had to change all my calls to give the color as a plain argument rather than keyword, which is fine, but inconsistent with the [docs for mkPen](https://pyqtgraph.readthedocs.io/en/latest/style.html#line-fill-and-color).