[FIX] tools: properly verify the size of HTML nodes

Instead of checking the size of text only node, let the xml_translate verify it.

The size verification using etree was introduced at 40efdccc but was
probably too simple.
Since 45352512, this becomes redundant with the nonspace method which can
be extended.

The re.sub is still relevant as some terms may contain only non-alphanumeric and
use push_translation (e.g. '>=' as a selection or xml terms extracted by babel
in static repository)

closes odoo/odoo#29432