Form::getHttpData: Clarify return type

The method will always return an array when `htmlName` is not specified.
This will allow us to remove some PHPStan casts/checks in consuming projects.

src/Forms/Form.php

@@ -457,6 +457,7 @@ public function setSubmittedBy(?SubmitterControl $by): static
 
 	/**
 	 * Returns submitted HTTP data.
+	 * @return ($htmlName is null ? array<mixed> : string|array<mixed>|Nette\Http\FileUpload|null)
 	 */
 	public function getHttpData(?int $type = null, ?string $htmlName = null): string|array|Nette\Http\FileUpload|null
 	{