I've attached a patch which I believe fixes the issue.

The problem appears to be that, when imagemagick is installed and the
image width is unset, that the image will be created by an elisp
expression like

(create-file "some/image.png" 'imagemagick nil :width nil)

which doesn't work, maybe because imagemagick tries to create a 0-width
image.

The attached patch reverts to the old behavior of only using imagemagick
when width is non-nil, fixing the bug introduced by 48da60f47.