show something useful when preview is empty

data/static/js/bitties/pyrom-bitty.js

@@ -152,6 +152,8 @@ export default class {
     const ta = document.getElementById('babycode-content');
     const markup = ta.value.trim();
     if (markup === '' || markup === this.#previousMarkup) {
+      previewErrorsContainer.textContent = 'Type something!';
+      previewContainer.textContent = '';
       return;
     }
     const bannedTags = JSON.parse(document.getElementById('babycode-banned-tags').value);