[FIX] web_editor: should not remove tag on backspace in nested li

Before this commit:

When we create a nested list and change a tag of the one of
the list item in nested list and press backspace from start it removes the tag.

After this commit:

Now pressing backspace from the start tag, the tag is not removed.

task - 3147461

closes odoo/odoo#115695

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