Fix Date-Picker for new meeting

[Elblinator]

Description:
While creating a new meeting the date-picker should initially be empty.
Currently the date-picker has the current date selected

Reproduction:

1. create a new meeting
2. look at the field "Meeting Date" (the date is filled with the current date)
3. if you start chossing a date the current date is preselected and "clear" has to be activated to empty the date

Wanted Description:
the date which is picked should be empty to begin with