misc: improve mapping performance for nullable union type

This adds a check within the loop for union type mapping, which will
skip `NullType` as it was already handled in `CasterProxyNodeBuilder`.

This greatly improves performance as it removes a useless call to the
whole node building process.

src/Mapper/Tree/Builder/UnionNodeBuilder.php

@@ -13,6 +13,7 @@
 use CuyZ\Valinor\Type\ScalarType;
 use CuyZ\Valinor\Type\StringType;
 use CuyZ\Valinor\Type\Types\InterfaceType;
+use CuyZ\Valinor\Type\Types\NullType;
 use CuyZ\Valinor\Type\Types\ShapedArrayType;
 use CuyZ\Valinor\Type\Types\UnionType;
 
@@ -38,6 +39,16 @@ public function build(Shell $shell, RootNodeBuilder $rootBuilder): TreeNode
         $all = [];
 
         foreach ($type->types() as $subType) {
+            // Performance optimisation: a `NullType` only accepts a `null`
+            // value, in which case the `CasterProxyNodeBuilder` would have
+            // handled it already. We can safely skip it here.
+            //
+            // @infection-ignore-all / This is a performance optimisation, so we
+            // cannot easily test this behavior.
+            if ($subType instanceof NullType) {
+                continue;
+            }
+
             $node = $rootBuilder->build($shell->withType($subType));
 
             if (! $node->isValid()) {

tests/Integration/Mapping/UnionMappingTest.php

@@ -58,6 +58,12 @@ public function test_union_mapping_works_properly_with_flexible_casting_enabled(
 
     public static function union_mapping_works_properly_data_provider(): iterable
     {
+        yield 'nullable scalar' => [
+            'type' => 'string|null',
+            'source' => null,
+            'assertion' => fn (mixed $result) => self::assertNull($result),
+        ];
+
         yield 'string or list of string, with string' => [
             'type' => 'string|list<string>',
             'source' => 'foo',