Fix handling registry image 404

[oliversun9]

The caller of the modified function:

	registryImage, imageID, err := fetchRegistryImageAndImageID(p)
	if err != nil {
		return release.PluginRelease{}, false, err
	}
	if registryImage == "" || imageID == "" {
		c.logger.InfoContext(ctx, "unable to detect registry image and image ID",
			slog.String("owner", identity.Owner()),
			slog.String("plugin", identity.Plugin()),
			slog.String("version", p.PluginVersion),
		)
		return release.PluginRelease{}, true, nil
	}

So we need fetchRegistryImageAndImageID to be able to return "", "", nil

[pkwarren]

This is a good fix however the underlying issue is that this CI workflow has been hung for ~22 hours: https://github.com/bufbuild/plugins/actions/runs/32984329655

Unfortunately it is unable to be canceled. We can merge this but I'm going to disable the release workflow until we can get a build going again.