image.py: get_histogram() will fail if image initialized with Nan values

[RussBerg]

if an image that will be populated as the pixel data is collected during a raster scan is initialized with Nan instead of an integer value
get_histogram() when np.histogram() is called will fail with:

<class 'ValueError'>:
autodetected range of [nan, nan] is not finite.

https://github.com/PierreRaybaut/guiqwt/blob/d3da7bacc47d66bafd7f69ed7e13b19742dc57d8/guiqwt/image.py#L746