CtVideo::Image buffer python binding with empty buffer segfault

When reading the video_last_image from a Device Server that has just been started, LimaCCDs segfault.

It looks like PyString_FromStringAndSize(NULL, 0) returns an invalid value which usage might segfault with Python 3.10.

The issue is seen in the Device Server that does not check if video.getLastImage() returns a proper value before parsing it:

    def read_video_last_image(self, attr):
        video = self.__control.video()
        self._videoStr = _video_image_2_struct(video.getLastImage())
        attr.set_value("VIDEO_IMAGE", self._videoStr)

Funnily I cannot reproduce this with the simulator, but it is systematic with the PCO.