[FIX] web_editor: ctrl+a container should be an element

If somehow there is no selection but the `_handleShortcuts` handler still gets
called with `CTRL+A`, `container` would be a JQUERY object instead of an
element leading to a crash in `selectNodeContents` which expects a `Node`.

task-3506666

closes odoo/odoo#136289

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>