Use Akeneo value if label is not defined for a locale

[lruozzi9]

Changes proposed in this pull request:

• Use Akeneo value if label is not defined for a locale in product option value handler

@webgriffe/wg-devs what do you think about this PR ?

[LucaGallinari]

With the API i cannot replicate this case. It seems to never return "null" as label value for a locale, instead it seems to never return the locale in the "labels" node.
Example: in a project with 2 locales it_IT and en_US, the "adesivo" attribute has a label value only for "it_IT" and this is the response of the "/attributes" API:

{
   ...
   "labels": {
      "it_IT": "Adesivo"
   },
   ...
}

[lruozzi9]

With the API i cannot replicate this case. It seems to never return "null" as label value for a locale, instead it seems to never return the locale in the "labels" node. Example: in a project with 2 locales it_IT and en_US, the "adesivo" attribute has a label value only for "it_IT" and this is the response of the "/attributes" API:

{
   ...
   "labels": {
      "it_IT": "Adesivo"
   },
   ...
}

It looks different behavior depending on the version of the PIM. In the v5.0.42 it returns null, but not in the v5.0.12. We have to investigate on it on a new issue.