Zero-net standard-rated lines mutate VAT rate to 0% during XML serialization

[douglasward]

Summary

Secretariat::LineItem#to_xml currently changes tax_percent to 0 when net_amount is zero:

if net_price&.zero?
  self.tax_percent = 0
end

This makes zero-value standard-rated invoice lines serialize as 0% VAT even when the supplied tax category/rate is standard VAT, for example 19%.

Why this matters

A fully discounted or zero-consideration line can still be a standard-rated taxable item. In that case the taxable base and tax amount are 0.00, but the VAT category/rate should remain the applicable rate, for example S / 19.00, not 0.00.

Changing the rate to zero conflates standard-rated zero-value lines with zero-rated or non-taxable lines.

Expected behavior

If a caller supplies:

• tax_category: :STANDARDRATE
• tax_percent: 19
• net_amount: 0
• tax_amount: 0

the XML should preserve RateApplicablePercent as 19.00.

Actual behavior

LineItem#to_xml mutates tax_percent to 0, so the generated XML contains RateApplicablePercent as 0.00.

Suggested fix

Avoid mutating tax_percent for zero-net lines. If some profiles need zero-rate normalization, make it opt-in or tied to an explicit zero-rated/untaxed tax category rather than inferred from net_amount == 0.

Environment

Observed with secretariat 3.9.0.
Relevant commit: 321657a80a71c08c0fb82b3865622a92e2cd49e1.