When we had only one sign existing (+ or -) and tried to create
again a tag with the same name we didn't reused the existing one and
recreated two new ones.
This situation can happen after the archive/unlink of tax tags when we delete report lines
with tags still in use.
This PR improves this behavior.

closes odoo/odoo#111315

Task: 3162014
Related: #107705
Signed-off-by: Olivier Colson (oco) <oco@odoo.com>