TimestampDatetimeWidget::massageFormValues does not allow NULL value

Problem/Motivation

When using the field widget TimestampDatetimeWidget for a timestamp field on an entity form and the timestamp field is left empty, on form submit the field will be populated with the current time.

Steps to reproduce

Add a timestamp field to a content type, and use the TimestampDatetimeWidget to render the form widget for the new field. Create an new content and leave the timestamp field empty.

Proposed resolution

Allow NULL value on a timestamp field

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet