date input: use distinct 'null' value state if unset by user

src/lib/DateInput.svelte

@@ -16,7 +16,7 @@
 
 	interface Props {
 		name?: string;
-		value?: CalendarDate;
+		value?: CalendarDate | null;
 		min?: CalendarDate;
 		// max?: CalendarDate; // TODO: Implement validation.
 		label?: string;
@@ -28,7 +28,7 @@
 
 	let {
 		name,
-		value = $bindable<CalendarDate | undefined>(),
+		value = $bindable<CalendarDate | null>(null),
 		/** min specifies lower bounds for the date input (WARNING: NOT IMPLEMENTED) */
 		min = new CalendarDate(1900, 0, 1),
 		/** max specifies upper bounds for the date input (WARNING: NOT IMPLEMENTED) */
@@ -108,7 +108,7 @@
 				if (value) {
 					setPrevious();
 					previousYearValue = undefined;
-					value = undefined;
+					value = null;
 				}
 				return;
 			}
@@ -121,7 +121,7 @@
 				if (value) {
 					setPrevious();
 					previousMonthValue = undefined;
-					value = undefined;
+					value = null;
 				}
 				return;
 			}
@@ -134,7 +134,7 @@
 				if (value) {
 					setPrevious();
 					previousDayValue = undefined;
-					value = undefined;
+					value = null;
 				}
 				return;
 			}