[FIX] account_edi_ubl_cii: tax exempt export

Previously, exporting BIS3 when there is a 0% tax in the invoice will
results in the XML showing warning of UBL-CR-601. But if we do not
include any TaxExemptionReason reason at all, a fatal error BR-E-10 will
pop up.

Here is the details of those 2 rules:

```xml
(with context: /*/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory
[normalize-space(cbc:ID) = 'E'][cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT'])
      <assert id="BR-E-10" flag="fatal" test="exists(cbc:TaxExemptionReason) or
        exists(cbc:TaxExemptionReasonCode)">
          [BR-E-10]-A VAT breakdown (BG-23) with VAT Category code (BT-118)
          "Exempt from VAT" shall have a VAT exemption reason code (BT-121)
          or a VAT exemption reason text (BT-120). </assert>

and

(no context)
<assert id="UBL-CR-601" flag="warning" test="
  not((cac:InvoiceLine|cac:CreditNoteLine)/cac:Item/
       cac:ClassifiedTaxCategory/cbc:TaxExemptionReason)">
          [UBL-CR-601]-A UBL invoice should not include the InvoiceLine
          Item ClassifiedTaxCategory TaxExemptionReason </assert>
```

Based on these rules, we can conclude that:
- TaxExemptionReason must not appear in InvoiceLine/Item/ClassifiedTaxCategory
- TaxExemptionReason must appear (when some line in invoice has exempt tax)
  in TaxTotal/TaxSubtotal/TaxCategory

Previously, the TaxExemptionReason will appear on both places.
This commit fixes that and adds a test to ensure that when a 0% tax is
present, TaxExemptionReason only appear in TaxTotal/TaxSubtotal/TaxCategory

closes odoo/odoo#154232

Task-id: 3703206
Signed-off-by: Laurent Smet (las) <las@odoo.com>
Signed-off-by: Yosua Nicolaus (yoni) <yoni@odoo.com>