Allow the input to be configured with no default value

[theconorodonnell]

Is your feature request related to a problem? Please describe.
It is currently not possible to configure this form input with no default value. The form builder inferface requires the editor to select a default value for the calendar input to be able to save the form configuration.

Describe the solution you'd like
Ideally when adding form inputs to a form, the input can be placed without any default value required. Having the default value populated on the calendar input can result in incorrect data submitted by the user. If the input has a value, and is required for submittion, the user can easily overlook changing this value resulting in inaccurate data being submitted.

Describe alternatives you've considered
Creating a new custom calendar form control for kentico 13. Seems like a waste of resources when this calendar picker has been developed by kentico.