Removed `cannotBeEmpty()` check from `fpn_tag.model` configuration key

[webhdx]

Description

cannotBeEmpty() validation check doesn't work with arrayNodes and throws an error with Symfony 3.4(dev).

[bluntelk]

Hi Fabien,

Now that Symfony 3.4 is well and truly out in the wild, is there any chance that this can be merged so we can avoid the deprecated warning this throws?

Thanks!

[epoplive]

Hi Fabien, I would also like to see this merged, this bundle errors in Symfony 4 during composer functions without this fix.