[backport 10.2] Saving an empty date field should always result in a placeholder value being set

As discussed in topic: https://groups.google.com/forum/#!topic/hippo-community/6P_ilQ6ftGc

When a new document containing an empty date is saved through the CMS, the date property on the node is filled with a placeholder value 0001-01-01T13:00:00.000+01:00. However, when a date already has a value and is subsequently emptied through the CMS, then the date property is removed from the node when the document is saved. This is inconsistent behaviour: in both cases an empty date is saved, so in both cases the resulting date value on the node should be equal. As a result, developers have to manage both cases separately in their code where they shouldn't have to.

Suggestion: change the behaviour so that the placeholder is always used when saving an empty date.