Remove user infos creates a different birth date in the front- and backend

[rich20]

To Reproduce

1. Mark the option "Delete all the profile info" before you ban a user
2. Check the profile infos in the frontend now (he get this birthdate 11/30/-0001)
3. Check the profile infos now in the backend, here is the birthdate 0000-00-00
4. Unban this user and try to save something in his profile

Actual result
You get this error, if you not edit the birthdate to a valid date
DateTime::__construct(): Failed to parse time string (11/30/-0001) at position 5 (/): Unexpected character

System information (please complete the following information)

Joomla version: 3.8.10 and 3.8.11-dev
Kunena version: 5.1.2-DEV
Php version: 7.2.1 and 7.0.25
Database version:

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.