[FIX] web_editor: prevent undefined class/style

The `_wrap` util function of `convert_inline` was not checking if
`className` and `style` were defined before adding them to the wrapper
element, leading to things like `<div class="undefined">element</div>`.

X-original-commit: 9fd6e069d07c3f7964511af7289d95c970d11b3e
Part-of: odoo/odoo#124465

addons/web_editor/static/src/js/backend/convert_inline.js

@@ -1623,8 +1623,12 @@ function _normalizeStyle(style) {
  */
  function _wrap(element, wrapperTag, wrapperClass, wrapperStyle) {
     const wrapper = document.createElement(wrapperTag);
-    wrapper.className = wrapperClass;
-    wrapper.style.cssText = wrapperStyle;
+    if (wrapperClass) {
+        wrapper.className = wrapperClass;
+    }
+    if (wrapperStyle) {
+        wrapper.style.cssText = wrapperStyle;
+    }
     element.parentElement.insertBefore(wrapper, element);
     wrapper.append(element);
     return wrapper;