ENYO-409: Handle case where date picker has not had a value set yet.

### Issue
In the `DatePickerSample`, if the `moon.DatePicker` has not yet been expanded, which means it does not currently have a value, pressing the "Set Date" button when any of the input fields (day, month, year) are empty will thrown an exception. This is caused by how the sample is implemented, which uses the relevant date part of the current value of the `moon.DatePicker` for the respective input field that is empty.

### Fix
We default to the current date as our baseline date if the `moon.DatePicker` does not have a current value.

### Notes
Please note that https://github.com/enyojs/moonstone/pull/1736 needs to be merged in order for the `DatePickerSample` to load.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>