frontend: Workflowitem with no due date shows invalid date in new datepicker

[SamuelPull]

An empty date is shown as an invalid date in the new date picker

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Expected Behavior 🤔

No date should show empty line, not error.

Steps to Reproduce 🕹

Steps:

1. Create a workflowitem with no due date
2. Edit that workflowitem
3. Bim 💣
4. ☠️

Context 🔦

Your Environment 🌎

Tech	Version
Frontend	v1.?.?
API	v1.?.?
Blockchain	v1.?.?
Export-Service	v1.?.?
Email-Service	v1.?.?
Chrome-Browser	?
etc.

[SamuelPull]

We cannot just override invalidDateMessage property, that would enable meaningless dates such as "asdf".
Probably need to override formatting function? https://material-ui-pickers.dev/api/KeyboardDatePicker