Fix exception when timezone is false

[Seb33300]

When passing false in the timezone param, twig returns an exception:

An exception has been thrown during the rendering of a template ("Twig\Extra\Intl\IntlExtension::createDateFormatter(): Argument 5 ($timezone) must be of type ?DateTimeZone, bool given

Passing false is supported and allows to skip the timezone conversion.

Regression introduced by #3903

[fabpot]

Thank you @Seb33300.