fix: ensure code exists inside pre before adding clipboard

quartz/components/scripts/clipboard.inline.ts

@@ -7,6 +7,7 @@ document.addEventListener("nav", () => {
   const els = document.getElementsByTagName("pre")
   for (let i = 0; i < els.length; i++) {
     const codeBlock = els[i].getElementsByTagName("code")[0]
+    if (codeBlock) {
       const source = codeBlock.innerText.replace(/\n\n/g, "\n")
       const button = document.createElement("button")
       button.className = "clipboard-button"
@@ -28,4 +29,5 @@ document.addEventListener("nav", () => {
       })
       els[i].prepend(button)
     }
+  }
 })