Cast total to string before setting value in refund inputs

[Prometee]

Q	A
Branch?	2.0
Bug fix?	yes
New feature?	no
Related tickets	none

Setting a value on an input field requires it to be a string. Without this fix you can end up with this error:

Behat\Testwork\Call\Exception\FatalThrowableError: Type error: Symfony\Component\Panther\DomCrawler\Field\InputFormField::setTextValue():
Argument #1 ($value) must be of type ?string, float given,
called in vendor/symfony/panther/src/DomCrawler/Field/InputFormField.php
on line 29 in vendor/symfony/panther/src/DomCrawler/Field/FormFieldTrait.php:57
Stack trace:
#0 vendor/symfony/panther/src/DomCrawler/Field/InputFormField.php(29): Symfony\Component\Panther\DomCrawler\Field\InputFormField->setTextValue(10.0)
#1 vendor/robertfausk/mink-panther-driver/src/PantherDriver.php(559): Symfony\Component\Panther\DomCrawler\Field\InputFormField->setValue(10.0)
#2 vendor/friends-of-behat/mink/src/Element/NodeElement.php(105): Behat\Mink\Driver\PantherDriver->setValue('((//html/descen...', 10.0)
#3 vendor/sylius/refund-plugin/tests/Behat/Page/Admin/OrderRefundsPage.php(47): Behat\Mink\Element\NodeElement->setValue(10.0)
#4 vendor/sylius/refund-plugin/tests/Behat/Context/Ui/RefundingContext.php(54):