Guard against undefined value in CodeMirror component (#8708)

When value is undefined/null, CodeMirror crashes trying to read
.length on it during EditorState.create. Return an empty editor
div instead.

Closes #8692

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/javascript/components/misc/CodeMirror.tsx

@@ -51,6 +51,10 @@ export default function CodeMirror({
   const [textarea, setTextarea] = useState<HTMLDivElement | null>(null)
   const viewRef = useRef<EditorView | null>(null)
 
+  if (value === undefined || value === null) {
+    return <div className="editor" />
+  }
+
   const setValue = (text: string) => {
     if (!viewRef.current) {
       return