string_utils library Null safety

1. equizz
2. string_utils library

equizz package

1. Libraries
2. API_manager
3. buttons
4. category
5. colors
6. displayers
7. drop_downs
8. firebase_options
9. game_page
10. game_recap_page
11. game_recap_stats
12. general_information
13. local_auth_service
14. login_page
15. main
16. main_menu_page
17. navigators
18. new_game_page
19. player_database_stats
20. player_info
21. question
22. questionnaire
23. ranking_page
24. register_page
25. result
26. separators
27. sizes
28. snack_bar
29. string_utils
30. text_fields
31. texts
32. user
33. user_profile_page