Merge branch 'ignacio-mf-master'

.travis.yml

@@ -2,6 +2,7 @@ language: php
 php:
   - '7.3'
   - '7.4'
+  - '8.0'
 
 install:
   - composer self-update

src/RijksregisternummerHelper.php

@@ -32,7 +32,7 @@ class RijksregisternummerHelper
    */
   public static function clean(?string $rijksregisternummer, string $formattingCharacters = '/[\.\-\ ]/'): string
   {
-    return preg_replace($formattingCharacters, '', $rijksregisternummer);
+    return preg_replace($formattingCharacters, '', $rijksregisternummer ?? '');
   }
 
   //--------------------------------------------------------------------------------------------------------------------

test/RijksregisternummerHelperTest.php

@@ -55,6 +55,25 @@ public function testClean05(): void
   {
     self::assertSame('66041066600', RijksregisternummerHelper::clean("660 41\n0666-00\x08", '/\D/'));
   }
+
+  //--------------------------------------------------------------------------------------------------------------------
+  /**
+   * Test clean with register number with null.
+   */
+  public function testClean06(): void
+  {
+    self::assertSame('', RijksregisternummerHelper::clean(null));
+  }
+
+  //--------------------------------------------------------------------------------------------------------------------
+  /**
+   * Test clean with register number with null.
+   */
+  public function testClean07(): void
+  {
+    self::assertSame('0', RijksregisternummerHelper::clean('0'));
+  }
+
   //--------------------------------------------------------------------------------------------------------------------
   /**
    * Test computeCheckDigits.