BF: Prevent exception on ImageStim.setImage(undefined)

[jalfje]

If the image of an ImageStim is based off a column in a data file, and an entry in that column is blank because there is not supposed to be an image, then the experiment crashes.

At present, ImageStim can handle having an undefined image (a) initially, and (b) when setImage(undefined) is called while the image is already undefined. In the second case, it handles the onChanged signal gracefully, because it checks whether existingImage is valid before accessing existingImage.src or image.src. However, if existingImage is valid but image is not (because you're trying to clear the image by intentionally setting it to undefined), then the check is insufficient and we get an unhandled exception on image.src.

This also changes the hasChanged logic to be false if both existingImage and image are undefined - but I think whether that should be the case is a design consideration. I don't have any sense of what _onChange is needed for, so I don't know which functionality is more appropriate. Feel free to revert to just : true; if it doesn't make sense.