Fix: null values are removed from the generated xml.

src/Xml/XMLWriter.php

@@ -22,6 +22,10 @@ public function __construct()
 
     protected function build(DOMNode $parent, string $nodeName, mixed $nodeValue, $namespace = null): void
     {
+        if (is_null($nodeValue)) {
+            return;
+        }
+
         if ($nodeValue instanceof Type) {
             $this->buildType($parent, $nodeName, $nodeValue);
             return;

tests/XmlNullValuesTest.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace Tests;
+
+use Firebed\AadeMyData\Enums\ExpenseClassificationCategory;
+use Firebed\AadeMyData\Enums\IncomeClassificationCategory;
+use Firebed\AadeMyData\Enums\IncomeClassificationType;
+use Firebed\AadeMyData\Models\ExpensesClassification;
+use Firebed\AadeMyData\Models\IncomeClassification;
+use Firebed\AadeMyData\Models\Invoice;
+use Firebed\AadeMyData\Models\InvoiceDetails;
+use PHPUnit\Framework\TestCase;
+use Tests\Traits\HandlesInvoiceXml;
+
+class XmlNullValuesTest extends TestCase
+{
+    use HandlesInvoiceXml;
+
+    public function test_null_values_are_removed_from_generated_xml()
+    {
+        $icls1 = new IncomeClassification();
+        $icls1->setClassificationType(null);
+        $icls1->setClassificationCategory(IncomeClassificationCategory::CATEGORY_3);
+        $icls1->setAmount(0);
+
+        $icls2 = new IncomeClassification();
+        $icls2->setClassificationType(IncomeClassificationType::E3_106);
+        $icls2->setClassificationCategory(IncomeClassificationCategory::CATEGORY_1_3);
+        $icls2->setAmount(0);
+
+        $ecls = new ExpensesClassification();
+        $ecls->setClassificationType(null);
+        $ecls->setClassificationCategory(ExpenseClassificationCategory::CATEGORY_2_1);
+        $ecls->setAmount(0);
+
+        $row = new InvoiceDetails();
+        $row->addIncomeClassification($icls1);
+        $row->addIncomeClassification($icls2);
+        $row->addExpensesClassification($ecls);
+
+        $invoice = new Invoice();
+        $invoice->addInvoiceDetails($row);
+
+        $classifications = $this->toXML($invoice)->InvoicesDoc->invoice->invoiceDetails;
+
+        $income = $classifications->incomeClassification;
+
+        $this->assertCount(2, $income);
+        $this->assertCount(2, $income[0]);
+
+        $this->assertEquals(IncomeClassificationCategory::CATEGORY_3->value, $income[0]->get('icls:classificationCategory'));
+        $this->assertEquals(0, $income[0]->get('icls:amount'));
+        $this->assertArrayNotHasKey('icls:classificationType', $income[0]);
+
+        $this->assertEquals(IncomeClassificationCategory::CATEGORY_1_3->value, $income[1]->get('icls:classificationCategory'));
+        $this->assertEquals(0, $income[1]->get('icls:amount'));
+        $this->assertEquals(IncomeClassificationType::E3_106->value, $income[1]->get('icls:classificationType'));
+
+        $expenses = $classifications->expensesClassification;
+        $this->assertCount(2, $expenses);
+
+        $this->assertEquals(ExpenseClassificationCategory::CATEGORY_2_1->value, $expenses->get('ecls:classificationCategory'));
+        $this->assertEquals(0, $expenses->get('ecls:amount'));
+        $this->assertArrayNotHasKey('ecls:classificationType', $expenses);
+    }
+}