Bug fix, prevent displaying 0000-00-00 00:00:00 as anything else in admin grids

[AntonEvers]

Description

The datetime 0000-00-00 00:00:00 is converted to a different time zone, leading to unexpected outcome. This string should be treated equal as NULL.

Fixed Issues (if relevant)

1. none

Manual testing scenarios

1. Add a '0000-00-00 00:00:00' date in the database in one of the sales grids
2. Set the time zone to something exotic
3. See that the 0000 time is turned into something meaningless, a random-ish date and time

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)

[orlangur]

Is it used anywhere besides deprecated \Magento\Sales\Model\ResourceModel\AbstractGrid::getLastUpdatedAtValue method?

[hostep]

Not really related, but a similar problem can happen during the generation of the sitemap.xml files: #9151

[AntonEvers]

@orlangur lots of ui column xml files use it: https://github.com/magento/magento2/search?utf8=%E2%9C%93&q=%22Magento%5CUi%5CComponent%5CListing%5CColumns%5CDate%22&type=