Don't error when a playlist has no modification date

[Abestanis]

Use the creation date if no modification date is present.

This fixes the following error observed on Crashlytics ([1], maybe also [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]):

Stacktrace

          Fatal Exception: io.flutter.plugins.firebase.crashlytics.FlutterError: type 'Null' is not a subtype of type 'int' in type cast
       at new Playlist.fromMap(playlist.dart:115)
       at ListIterable.toList(dart:_internal)
       at ContentControl.refetch(content.dart:409)
       at Future.wait.<fn>(dart:async)
       at ContentControl.refetchSongsAndPlaylists(content.dart:682)
       at ContentControl.createPlaylist(content.dart:739)
       at ShowFunctions.showCreatePlaylist.submit(show_functions.dart:64)