The Date and Time attribute displays 1st january 1970 when no date is set.
This makes it impossible to have empty datetime's, even when the attribute is marked as not required.
This issue only happens as of eZ Publish 5.3
*Steps to reproduce:*
1. Create a new class "TestDateTime" with a "Date and Time" attribute
2. Create a content object of type "TestDateTime" and notice that the "Date and Time" field is already filled with 1970 01 01 1:00 (screenshot attached)