Fix :xhp::__construct $children type

Hack now typechecks XHP constructors (facebook/hhvm@af6f350)
which revealed this error.

Author: jjergus

src/core/ComposableElement.php

@@ -35,7 +35,7 @@ protected function init(): void {
    */
   final public function __construct(
     KeyedTraversable<string, mixed> $attributes,
-    Traversable<XHPChild> $children,
+    Traversable<?XHPChild> $children,
     dynamic ...$debug_info
   ) {
     parent::__construct($attributes, $children);

src/core/XHP.php

@@ -16,7 +16,7 @@ abstract class :xhp implements XHPChild, JsonSerializable {
 
   public function __construct(
     KeyedTraversable<string, mixed> $attributes,
-    Traversable<XHPChild> $children,
+    Traversable<?XHPChild> $children,
   ): void {
   }
   abstract public function appendChild(mixed $child): this;

tests/BasicsTest.php

@@ -50,6 +50,11 @@ public function testXFrag(): void {
     expect($xhp->toString())->toEqual('<div>herpderp</div>');
   }
 
+  public async function testScalarChildren(): Awaitable<void> {
+    $xhp = <div>{42}{' str '}{3.14}{null}</div>;
+    expect($xhp->toString())->toEqual('<div>42 str 3.14</div>');
+  }
+
   public function testEscaping(): void {
     $xhp = <div>{"foo<SCRIPT>bar"}</div>;
     expect($xhp->toString())->toEqual('<div>foo&lt;SCRIPT&gt;bar</div>');