fix(build): don't warn on blank lang in fences

src/node/markdown/plugins/highlight.ts

@@ -86,6 +86,7 @@ export async function highlight(
     lang =
       lang.replace(lineNoRE, '').replace(vueRE, '').toLowerCase() || defaultLang
 
+    if (lang) {
       const langLoaded = highlighter.getLoadedLanguages().includes(lang as any)
       if (!langLoaded) {
         console.warn(
@@ -97,6 +98,7 @@ export async function highlight(
         )
         lang = defaultLang
       }
+    }
 
     const lineOptions = attrsToLines(attrs)
     const cleanup = (str: string) =>