Better handling of date inputs within forms

- use datetime_picker_formfield
- Allow null / empty dates

lib/widget/home.dart

@@ -94,7 +94,7 @@ class _InvenTreeHomePageState extends State<InvenTreeHomePage> {
     Navigator.push(
       context,
       MaterialPageRoute(
-        builder: (context) => PurchaseOrderListWidget()
+        builder: (context) => PurchaseOrderListWidget(filters: {})
       )
     );
   }