Update to ng-formworks release 19.5.6
### Feature Description

### What is happening?

Fields with format `date-time` in config-schema.json return the string "null" if no value is entered.
This is a bug in ng-formworks, which is a Homebridge UI dependency.

### What you expect to happen?

Fields with format `date-time` should not return a value if user does not enter a value.

### What problems would this new feature solve?

ng-formworks release 19.5.6 fixes this bug
