Fix PHPStan issue with nette/component-model 3.1.0

nette/component-model 3.1.0 made the type annotations more precise,
correctly declaring that `null` will not be returned when `$throw` argument is `true` (default):
nette/component-model@fbab7bc

As a result PHPStan started to complain:

    Expression on left side of ?? is not nullable.

(cherry picked from commit 7445d0d, 2afcd08)

Author: jtojnar

src/Multiplier.php

@@ -11,6 +11,7 @@
 use Nette\Forms\Controls\BaseControl;
 use Nette\Forms\Controls\SubmitButton;
 use Nette\Forms\Form;
+use Nette\InvalidStateException;
 use Nette\Utils\ArrayHash;
 use Nette\Utils\Arrays;
 use Traversable;
@@ -399,10 +400,11 @@ protected function fillContainer(Container $container): void
 
 	/**
 	 * @return string[]
+	 * @throws InvalidStateException when not attached.
 	 */
 	protected function getHtmlName(): array
 	{
-		return explode('-', $this->lookupPath(Form::class) ?? '');
+		return explode('-', $this->lookupPath(Form::class));
 	}
 
 	protected function createContainer(): Container