[FIX] web_editor: fix select all in empty paragraph

The previous code with `parentsUntil` would not return any result
in an empty paragraph because the only valid element would be the
anchorNode itself but `parentsUntil` would only return parents and
never the base node itself. It makes sense, but is not suitable for
the case at hand.

Before this commit:
CTRL+A does nothing from inside an empty paragraph.

After this commit:
CTRL+A correctly triggers a select all from inside an empty paragraph.

Task-3150956

closes odoo/odoo#121711

X-original-commit: 4bcba1d7
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>
Co-authored-by: dhba-odoo <dhba@odoo.com>
Co-authored-by: David Monjoie <dmo@odoo.com>