Reroll #4051: Flexible DateTime widget: Small UX improvement when clearing date

[dafeder]

fixes #4051
replaces #4053

• Test coverage exists
• Documentation exists

QA Steps

• Clearing a date with time part "00:00:00" should allow to save and clear the time part while processing the value. Can use Release Date in dataset for example.

[dafeder]

This looks ready to merge for me, but github won't let me approve (I assume because I added a commit)? Passing along to someone else on the team to push the button...