Image syntax should no be triggered when the reference is empty

Small note that it's unlikely that filename would be used as an attachment name but the default attachment name can be configured (see https://extensions.xwiki.org/xwiki/bin/view/Extension/Model%20Module#HFromStringandEntityReference).

For example:

model.reference.default.attachment = image.png

However, I still agree that I doubt many xwiki admins will have configured this...