Fix timezone fallback to CoreExtension in IntlExtension

[keulinho]

This is probably a regression from #3844

Refer to my comment on the original MR: #3844 (comment)

[fabpot]

Thank you @keulinho.

[petski]

This PR breaks the behavior of format_datetime with timezone set to false. Calling format_datetime with timezone to false is documented as:

If the date is already a DateTime object, and if you want to keep its current timezone, pass false as the timezone value:
{{ datetime|format_datetime(locale='en', timezone=false) }}

The exception thrown if the above feature is used:

Twig\Extra\Intl\IntlExtension::createDateFormatter(): Argument #5 ($timezone) must be of type ?DateTimeZone, false given, called in /var/www/vendor/twig/intl-extra/IntlExtension.php on line 379

[xabbuh]

@petski see #3965