allow null values in CheckMissingCommand

[Guite]

This PR ensures that translation:check-missing does not break if a null value is given to the closure used in countEmptyTranslations method.

Error message:

In CheckMissingCommand.php line 137:
                                                                               
Argument 1 passed to Translation\Bundle\Command\CheckMissingCommand::Translation\Bundle\Command\{closure}() must be of the type string, null given