Resolve PHPStan level 6 findings

tests/unit/Mapper/StringMapperTest.php

@@ -18,21 +18,21 @@ use PHPUnit\Framework\TestCase;
 #[TestDox('String Mapper (Unit tests)')]
 class StringMapperTest extends TestCase
 {
-    public function testMapSucceedsWhenFieldIsPresentAndString()
+    public function testMapSucceedsWhenFieldIsPresentAndString(): void
     {
         $result = ['test_field' => 'test_value'];
         $mapper = new StringMapper('test_field');
         $this->assertEquals('test_value', $mapper->map($result), 'String value not returned correctly');
     }
 
-    public function testMapSucceedsWhenFieldIsPresentAndNotString()
+    public function testMapSucceedsWhenFieldIsPresentAndNotString(): void
     {
         $result = ['a_number' => 6.7];
         $mapper = new StringMapper('a_number');
         $this->assertEquals('6.7', $mapper->map($result), 'Number value not returned correctly');
     }
 
-    public function testMapSucceedsWhenFieldIsNotPresent()
+    public function testMapSucceedsWhenFieldIsNotPresent(): void
     {
         $mapper = new StringMapper('something_else');
         $this->assertNull($mapper->map([]), 'Missing value not returned correctly');