Fixed getNormalizedIdentifier

in case `$values` is empty we was not returning null and it is a problem in the AdminBundle. The result of this function have to be null in empty case. The regression was caused by https://github.com/sonata-project/SonataPropelAdminBundle/commit/7847ba7c90ac15cbe3b6e03d159e85a9e7315cbc

@K-Phoen Can you review this changeset ? thx