fix(tasks): default reminder to current date

This fixes an issue where the default date for a new reminder was
1970-01-01 (unix timestamp 0). It was caused by a new date object being
created but since the reminder that was creatd was new, this was created
as null date, which equals a 0 unix timestamp.

Resolves https://github.com/go-vikunja/vikunja/issues/359

frontend/src/components/tasks/partials/ReminderDetail.vue

@@ -144,7 +144,7 @@ watch(
 		if (newReminder) {
 			reminder.value = newReminder
 
-			if (newReminder.relativeTo === null) {
+			if (newReminder.relativeTo === null && newReminder.reminder !== null) {
 				reminderDate.value = new Date(newReminder.reminder)
 			}
 

frontend/src/components/tasks/partials/Reminders.vue

@@ -92,7 +92,7 @@ function updateData() {
 	})
 }
 
-function addNewReminder(newReminder: ITaskReminder) {
+function addNewReminder(newReminder: ITaskReminder|null) {
 	if (newReminder === null) {
 		return
 	}