Fix ZeroDivisionError in ImageStat

[radarhere]

Resolves #9104

If an image has zero dimensions, then ImageStat may raise a ZeroDivisionError.

>>> from PIL import Image, ImageStat
>>> im = Image.new("RGB", (0, 0))
>>> stat = ImageStat.Stat(im)
>>> stat.count
[0, 0, 0]
>>> stat.rms
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "functools.py", line 993, in __get__
    val = self.func(instance)
  File "PIL/ImageStat.py", line 144, in rms
    return [math.sqrt(self.sum2[i] / self.count[i]) for i in self.bands]
  File "PIL/ImageStat.py", line 144, in <listcomp>
    return [math.sqrt(self.sum2[i] / self.count[i]) for i in self.bands]
ZeroDivisionError: float division by zero

This just returns zero values for this scenario instead.