Update flutter_touch_spin to null safe version

Author: danvick

pubspec.yaml

@@ -18,7 +18,7 @@ dependencies:
  flutter_colorpicker: ^0.4.0
  flutter_chips_input: ^1.9.5
  flutter_datetime_picker: ^1.5.0
- flutter_touch_spin: ^2.0.0-nullsafety.0
+ flutter_touch_spin: ^2.0.0-nullsafety.1
  flutter_typeahead: ^3.1.0
  intl: ^0.17.0
  rating_bar: ^0.2.0