It should be possible to use swupd-image.bbclass with an image without requiring that the image also have additional bundles (on top of the os-core/base image) defined.
Fixed building images when SWUPD_BUNDLES is empty with: http://git.yoctoproject.org/cgit/cgit.cgi/meta-swupd/commit/?id=8bfd5adc5ce48cf80f4d2369cd843841dcc11dd8 and added some logic to prevent mega images being needlessly created in this scenario: http://git.yoctoproject.org/cgit/cgit.cgi/meta-swupd/commit/?id=b45ff4ff8389431f80c744cb17a9019bae512d44