Don't crash if target of image macro is unpublished

If the target of an image macro resolves to an image in the content catalog, but that image is unpublished, Antora crashes. This situation can occur if the filename or the folder its in begins with an _. In this case, the virtual file lacks the out and pub properties. Example:

image::_image-not-published.png[]

Since the image is not published, Antora should consider this to be an unresolved reference.