ra DateInput default value makes prisma update/create mutations crash

[aellanglet]

Since lasts ra updates, DateInput got a new behavior or more precisely a fix which add an empty string as default value.

This makes dataprovider crash because prisma doesn't accept "" as DateTime but only null value.

I did a simple fix and thought you might need it.

[macrozone]

check out if it works with our version: https://github.com/panter/ra-data-prisma