[Gallery Magick] Make the handling of zero dimensions same as Hippo Gallery's default behavior

Currently, during resizing, if an image's target dimension (width/height) is zero (0), the plugin will treat "as is". This is different from the default behavior of Hippo Gallery, where "A width or height of 0 or less means 'unbounded', and results in a bounding box that does not restrict scaling in either the width or height, respectively. When both width and height are 0 or less, the image is not scaled at all but merely copied."