Feature/reset filter display

[Gorlac]

if datepicker's field value is empty, update field value with an empty string as default, rather than a empty object. This allow the resource store to correctly count current filters.

Resolving this issue:

I set a date in the picker, page is refreshed with correct results, filters count is displayed as 1

If I try to reset the date field to it's original value, by deleting the date manually, page is refreshed with correct results, but the filter set previously is still detected and counted, even if it's value doesn't interfer in the result. That's because the original value is an empty string, and the value returned by removing a date manually is an empty object.

The resource store does not detect it, and display the resource as filtered.