Material-ui-pickers: DatePicker: null value for maxDate and minDate

[hongyuan1306]

For DatePicker, the default value for minDate and maxDate is '1900-01-01' and '2100-01-01' respectively. When I set one of these values to null, I expected the control would take up the default value, because for most of the react components, setting a property to null would mean the same as not having it set.

However, DatePicker behaves a little unexpected. It prevents the selection of any date. I can not see any reason for this behavior and would suggest that setting minDate or maxDate to null shall mean the same as setting them to undefined and the same as not having set them.

[dmtrKovalenko]

Use undefined to make minDate fill with default value