[FIX] mail: handle parsing of void body in incoming emails

When body does not contain any tag or content parsing currently fails
with an ``lxml.etree.ParserError``. To avoid that we can improve condition
about void body: stripping void characters allows to avoid that traceback.

Task-2641572
PR odoo#76159
Closes odoo#75625

Part-of: odoo/odoo#76159
Co-authored-by: Thibault Delavallee <tde@odoo.com>