fix support for missing rustdoc CSS in header

Author: syphar

templates/rustdoc/head.html

@@ -6,7 +6,9 @@
           {%- let build_slug2 =  slug::slugify(crate::BUILD_VERSION) -%}
         {%- endif -%}
 
-        <link rel="stylesheet" href="/-/static/{{rustdoc_css_file.as_ref().expect("rustdoc_css_file missing")}}?{{build_slug2}}" media="all" />
+        {%- if let Some(css_file) = rustdoc_css_file -%}
+            <link rel="stylesheet" href="/-/static/{{css_file}}?{{build_slug2}}" media="all" />
+        {%- endif -%}
         <link rel="stylesheet" href="/-/static/font-awesome.css?{{build_slug2}}" media="all" />
 
         <link rel="search" href="/-/static/opensearch.xml" type="application/opensearchdescription+xml" title="Docs.rs" />