fix: build without any module scoped preview styles (#108)

assets/decap-cms/init.js.tmpl

@@ -2,13 +2,14 @@
 {{/* Register preview styles. */}}
 {{- $css := resources.Get "decap-cms/scss/index.scss.tmpl" | resources.ExecuteAsTemplate "decap-cms/scss/index.scss" $ctx }}
 {{- $css = $css | toCSS (dict "targetPath" "css/decap-cms.css" "outputStyle" (cond hugo.IsProduction "compressed" "")) }}
+{{- $css = slice $css }}
 {{- range $name, $partial := default slice site.Params.decap_cms._preview_style_partials }}
   {{- if not $partial.path }}
     {{- errorf "[decap-cms] preview style partial %q's %q is required." $name "path" }}
   {{- else if not (printf "partials/%s.html" $partial.path | templates.Exists) }}
     {{- errorf "[decap-cms] preview style partial %q does not exist: partials/%s.html." $name $partial.path }}
   {{- else }}
-    {{- $css = slice $css | append (partial $partial.path $ctx) }}
+    {{- $css = $css | append (partial $partial.path $ctx) }}
   {{- end }}
 {{- end }}
 {{- $css = $css | resources.Concat "css/decap-cms.css" | fingerprint }}