Fixed 'Non-numeric value' warning on account create/save when DOB field is visible

[vovayatsyuk]

Description

This commit fixes:

• broken customer registration page when "Date of Birth" field is visible on the frontend.
• not working customer save method when DOB is visible

Related Issues

• Customer with empty "Date of Birth" cannot be saved #12146

Manual testing scenarios

1. Navigate to Stores > Configuration > Customer Configuration > Name and Address Options and make DOB field visible.

2. Navigate to frontend Customer Registration page. An exception with warning will appear:

   Trace:

   Exception #0 (Exception): Warning: A non-numeric value encountered in vendor/magento/zendframework1/library/Zend/Locale/Math/PhpMath.php on line 94
   #0 vendor/magento/zendframework1/library/Zend/Locale/Math/PhpMath.php(94): Magento\Framework\App\ErrorHandler->handler(2, 'A non-numeric v...', '...', 94, Array)
   #1 vendor/magento/zendframework1/library/Zend/Locale/Format.php(385): Zend_Locale_Math_PhpMath::Sub('0', '', 0)
   #2 vendor/magento/zendframework1/library/Zend/Locale/Format.php(662): Zend_Locale_Format::toNumber('', Array)
   #3 vendor/magento/zendframework1/library/Zend/Filter/NormalizedToLocalized.php(106): Zend_Locale_Format::toFloat(NULL, Array)
   #4 lib/internal/Magento/Framework/Data/Form/Filter/Date.php(85): Zend_Filter_NormalizedToLocalized->filter(NULL)
   #5 app/code/Magento/Customer/Block/Widget/Dob.php(131): Magento\Framework\Data\Form\Filter\Date->outputFilter(NULL)
   #6 app/code/Magento/Customer/Block/Widget/Dob.php(97): Magento\Customer\Block\Widget\Dob->applyOutputFilter(NULL)
   #7 app/code/Magento/Customer/view/frontend/templates/form/register.phtml(32): Magento\Customer\Block\Widget\Dob->setDate(NULL)
   

3. Navigate to the backend, open existing customer and try to save it with empty DOB field. The page reloads with an error message "Something went wrong while saving the customer.".

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)