text: Title: Error when using FFT on data with one point
  Body: Activating the FFT transform on data with one point raises the following error:
  
  > File ~\anaconda3\lib\site-packages\pyqtgraph\graphicsItems\PlotDataItem.py:1190 in _fourierTransform
  uniform = not np.any(np.abs(dx-dx[0]) > (abs(dx[0]) / 1000.))
  IndexError: index 0 is out of bounds for axis 0 with size 0
  
  This occurs because `dx = np.diff(x)` is empty when x is of size 1.
  
  A possible solution is to add the following condition `if len(x) == 1: return np.array([0]), abs(y)` in the beginning of _fourierTransform in PlotDataItem.py.
  
   * PyQtGraph version: tested on 0.13.3 but will also raise same error in 0.13.7 has the relevant code has not changed.