Fix number validator empty values

[skybamar]

According to addInteger method in Nette\Forms\Container it seems that integer input is supposed to be nullable and therefore empty value should be valid input for integer (assuming also for float).

https://github.com/nette/forms/blob/35f9b84c0245640cca2c6aa5544d0de4bb06a9d8/src/Forms/Container.php#L300

[f3l1x]

Thanks.