JournalEditor: Avoid crashing if note has no creation date

In the future when we improve the git itegration that will never happen,
but for now, it's what we have.

lib/widgets/journal_editor_header.dart

@@ -9,6 +9,9 @@ class JournalEditorHeader extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    if (note.created == null) {
+      return Container();
+    }
     var dateStr = DateFormat('MMMM, yyyy').format(note.created);
     var timeStr = DateFormat('EEEE HH:mm').format(note.created);