Invalid datetime format when saving a non-triggered conditional date field

Steps to reproduce the bug:
- Create a required date field with a default start date and a required but empty by default end date
- Add this field to a node type and turn it into a conditional field triggered by another field
- Attempt to create a node without triggering the date field
=> Save should crash with error SQLSTATE[22007]: Invalid datetime format.

This does not happen if you trigger the date field, empty its start date, untrigger the field, then save.