Only use English slug if one exists (github#28399)

Author: rsese

lib/render-content/plugins/use-english-headings.js

@@ -19,7 +19,11 @@ export default function useEnglishHeadings({ englishHeadings }) {
       // get English slug
       const englishSlug = slugger.slug(englishHeading)
       // use English slug for heading ID and link
-      node.properties.id = englishSlug
+      if (englishSlug) {
+        // only use English slug if there is one, otherwise we'll end up with
+        // empty IDs
+        node.properties.id = englishSlug
+      }
     })
   }
 }