Attribute data source issue

[Nuranto]

Same error as #2605 reappeared.

If you have for example news_from_date :

• set to '2040-10-10' in default view
• set to (empty) in store view (NULL in database)

then, 2040-10-10 will be stored in store ES index, instead of an empty value. The product will be wrongly shown as new in that store view.

I'll provide another PR for this.