[FIX] website: prevent traceback when iframe has no src

Commit [1] introduced an cleaning of the iframefallback on page unload,
which was removing the `autoplay` attribute of the iframe `src` URL.
It was done to prevent video/music to keep playing in the invisible
iframe.
The fix was not generic and purposely targetted the elements which we
identified as problematic.

Commit [2] then made that cleaning generic, catching all iframe.
Doing so, it was actually catching too many iframes and led to traceback
on iframe not having a set `src`.
This is the case for every database having a configured reCaptcha, among
other things.
For reCaptcha, it's because it's adding the following iframe dynamically
through their third party script:
`<iframe style="display: none;"></iframe>`

This commit ensures we only target iframe having an URL attribute being
set to something.

Step to reproduce (with reCaptcha):
- Enable and configure reCaptcha
- Go to /contactus
- Leave the iframe page, eg click on "Contactus" menu to refresh it
-> Traceback

[1]: https://github.com/odoo/odoo/commit/ad78585cd514f5ff16647572d34937c18a112529
[2]: https://github.com/odoo/odoo/commit/6be8af36726d750065972e122e8f4a0aa0f56a17

opw-3343450

closes odoo/odoo#123621

X-original-commit: 463d4ee0c8000f948463772b7c96c8d111bcbc8e
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/website/static/src/client_actions/website_preview/website_preview.js

@@ -428,7 +428,7 @@ export class WebsitePreview extends Component {
     _cleanIframeFallback() {
         // Remove autoplay in all iframes urls so videos are not
         // playing in the background
-        const iframesEl = this.iframefallback.el.contentDocument.querySelectorAll("iframe");
+        const iframesEl = this.iframefallback.el.contentDocument.querySelectorAll('iframe[src]:not([src=""])');
         for (const iframeEl of iframesEl) {
             const url = new URL(iframeEl.src);
             url.searchParams.delete('autoplay');