fix(e-invoices): Handle zero-fee invoices in allowance calculation (#5287)

## Context
Invoices with zero-amount fees (e.g. yearly plans with only fixed
charges terminated mid-period) cause `Invoices::GenerateDocumentsJob` to
fail with `ArgumentError: must be initialized with a finite value` when
the billing entity has e-invoicing enabled.

Scenario to reproduce:
Preconditions:
- Organization billing entity with E-Invoicing enabled
- Plan with Subscription Fee amount = 0

Steps:
1. Add a subscription with the Plan to a customer
2. Terminate subscription
Actual result in Sidekiq: Invoices::GenerateDocumentsJob, ArgumentError:
must be initialized with a finite value

## Description
Root cause: If all fees are zero, `fdiv(0)` produced `Infinity` which
`Money.new` rejects.
Fix: Added a guard before the division when fees are zero.

Author: D1353L

app/serializers/e_invoices/invoices/common.rb

@@ -67,6 +67,8 @@ def taxes(&block)
       end
 
       def allowances_per_tax_rate
+        fees_total = invoice.fees.sum(:precise_amount_cents)
+
         invoice.fees.group_by(&:taxes_rate).map do |tax_rate, fees|
           total_amount = fees.sum(&:precise_amount_cents)
 
@@ -76,7 +78,8 @@ def allowances_per_tax_rate
 
             [tax_rate, total_amount - charged_amount]
           else
-            [tax_rate, total_amount.fdiv(invoice.fees.sum(:precise_amount_cents)) * allowances]
+            proportion = fees_total.zero? ? 0 : total_amount.fdiv(fees_total)
+            [tax_rate, proportion * allowances]
           end
         end.to_h
       end

spec/serializers/e_invoices/invoices/factur_x/builder_spec.rb

@@ -315,6 +315,22 @@
           expect(subject).to contains_xml_node("#{root}[3]/ram:CategoryTradeTax/ram:RateApplicablePercent").with_value("10.00")
         end
       end
+
+      context "when all fees have zero precise_amount_cents" do
+        let(:invoice) { create(:invoice, coupons_amount_cents: 100, invoice_type:) }
+        let(:invoice_fee1) { create(:fee, invoice:, taxes_rate: 0.0, precise_amount_cents: 0, taxes_precise_amount_cents: 0) }
+        let(:invoice_fee2) { nil }
+        let(:invoice_fee3) { nil }
+        let(:invoice_fee4) { nil }
+
+        it "does not raise an error" do
+          expect { subject }.not_to raise_error
+        end
+
+        it "does not contain SpecifiedTradeAllowanceCharge tags" do
+          expect(subject.xpath(root).length).to eq(0)
+        end
+      end
     end
 
     context "when SpecifiedTradePaymentTerms tag" do

spec/serializers/e_invoices/invoices/ubl/builder_spec.rb

@@ -221,6 +221,22 @@
           expect(subject).to contains_xml_node("#{root}[3]/cac:TaxCategory/cac:TaxScheme/cbc:ID").with_value("VAT")
         end
       end
+
+      context "when all fees have zero precise_amount_cents" do
+        let(:invoice) { create(:invoice, coupons_amount_cents: 100, invoice_type:) }
+        let(:invoice_fee1) { create(:fee, invoice:, taxes_rate: 0.0, precise_amount_cents: 0, taxes_precise_amount_cents: 0) }
+        let(:invoice_fee2) { nil }
+        let(:invoice_fee3) { nil }
+        let(:invoice_fee4) { nil }
+
+        it "does not raise an error" do
+          expect { subject }.not_to raise_error
+        end
+
+        it "does not contain AllowanceCharge tags" do
+          expect(subject.xpath(root).length).to eq(0)
+        end
+      end
     end
 
     context "when TaxTotal tag" do