[3.2][Validation] New ValidationCacheWarmer incorrectly handles empty validation.yml

[lbayerl]

As were using annotations for our validation meta data we don't use yaml validation definitions.
But we still have an empty validation.yml in OurBundle/Resources/config/ - this wasn't an issue until Symfony 3.2.

But obviously with the new ValidationCacheWarmer this file is being parsed and to be merged with other validations here:

symfony/src/Symfony/Component/Validator/Mapping/Loader/YamlFileLoader.php

Line 70 in 6bdaf0b

return array_keys($this->classes);

In our case we got an error that array_keys expects an array which clearly can't be the case here.

This issue might as well be related to #20739