[FIX] web_editor: ensure _makeHint receives an element

Make a hint in the editor only if the first element is an element,
otherwise the method `_makeHint` would crash.

task-2898432

Part-of: odoo/odoo#94875

addons/web_editor/static/src/js/editor/odoo-editor/src/OdooEditor.js

@@ -3372,7 +3372,7 @@ export class OdooEditor extends EventTarget {
         }
 
         // placeholder hint
-        if (this.editable.textContent === '' && this.options.placeholder) {
+        if (this.editable.textContent === '' && this.options.placeholder && this.editable.firstChild.innerHTML) {
             this._makeHint(this.editable.firstChild, this.options.placeholder, true);
         }
     }