Fix datetime type product that show current date when is empty in grids

[enriquei4]

Description

In the product grid, when a date attribute is listed and the value was empty, the current date was displayed. This was very confusing because I did not know if the value was assigned or not. Now you can see empty cells when is unassigned.

Fixed Issues (if relevant)

datetime type product attribute showing current date #9869

Manual testing scenarios

1. Create a date type product attribute
2. assign that product to the required attribute set
3. you can edit the attribute value in the product edit.
4. create a new product and simply not assign any value to this attribute.
5. save product
6. Now you dont see the current date if is unset.

NOW:

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[enriquei4]

@miguelbalparda Could I backport this to 2.2 and 2.1?

[miguelbalparda]

Yes please @enriquei4!