Error in Monitor Center when "Plot gamma" is clicked with no calibration data loaded

Minor issue but raises the following exception:

```
Traceback (most recent call last):
  File "C:\Program Files\PsychoPy3\lib\site-packages\psychopy\monitors\MonitorCenter.py", line 1028, in plotGamma
    if lumsPre.any() != None:
AttributeError: 'NoneType' object has no attribute 'any'
```