Symfony version(s) affected: 3.2 and later

Description
When using the DateIntervalType form type with the integer widget and using weeks rather than days, the following exception is thrown when rendering an interval containing exactly 0 weeks:

Unable to transform value for property path "[weeks]": Expected a string.

The exception is thrown in \Symfony\Component\Form\Extension\Core\DataTransformer\NumberToLocalizedStringTransformer, line 142.

The problem is that \Symfony\Component\Form\Extension\Core\DataTransformer\DateIntervalToArrayTransformer initialises the value for the weeks part of the interval to integer 0. If days is available, this will be updated to a numeric string, but if not it remains at integer 0.

if (in_array('weeks', $this->fields, true)) {
    $result['weeks'] = 0;
    if (isset($result['days']) && (int) $result['days'] >= 7) {
        $result['weeks'] = (string) floor($result['days'] / 7);
        $result['days'] = (string) ($result['days'] % 7);
    }
}

Ultimately this integer reaches NumberToLocalizedStringTransformer, where on line 141 a type check occurs:

if (!is_string($value)) {
    throw new TransformationFailedException('Expected a string.');
}

How to reproduce
Create a form with the following DateIntervalType child:

$builder->add('interval', DateIntervalType::class, [
    'widget'     => 'integer',
    'with_weeks' => true,
    'with_days'  => false,
]);

Set the form's data to new \DateInteval('PY1M1W0'). Render the form to produce the error.

Possible Solution
Change the default value for weeks to '0'. PR incoming shortly.