Form::isSubmitted() has `@return ISubmitterControl|false`, but should be `@return ISubmitterControl|bool`

[Grez]

• bug report? yes
• feature request? no
• version: 2.4.4

Description

If the submit button ceases to exist before processing the form, Form::isSubmitted() will return TRUE, but docs say it can only return ISubmitterControl|false

Steps To Reproduce

    protected function createComponentBlaForm()
    {
        $form = new Nette\Application\UI\Form();

        // easiest way to simulate disappearing button
        if (mt_rand(1, 2) === 1) {
            $form->addSubmit('send');
        } else {
            \Tracy\Debugger::barDump('Sending button does not exist');
        }

        $form->onSuccess[] = function (Nette\Application\UI\Form $form) {
            \Tracy\Debugger::barDump($form->isSubmitted());
        };

        return $form;
    }

Possible results after sending: