[FIX] web_editor: fix scrolling the page while dragging a snippet

Since the commit [1], it's no longer possible to scroll the page while
dragging a snippet thanks to its thumbnail.

This is because for mass mailing the scroll element is not the same and
to check it we were looking for a class in the DOM. For that we did a
jQuery 'find' but without a 'length', the jQuery 'find' is always
truthy.

[1]: https://github.com/odoo/odoo/commit/29c6c80ac7009aa5068c763778342f7c6b115fad



task-2798872

closes odoo/odoo#86745

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>