[sensors] Migrate to null safety (flutter-team-archive#3423)

[quick_actions] Migrate to null safety (flutter-team-archive#3421)

[shared_preferences] Migrate platform plugins to null-safety (flutter…

…-team-archive#3523)

Migrates shared_preferences_linux and shared_preferences_web to null-safety.

[file_selector_platform_interface] File selector nnbd (flutter-team-a…

…rchive#3509)

Migrating file_selector_interface to null-safety

[camera_platform_interface] Migrate to null safety (flutter-team-arch…

…ive#3497)

[camera_platform_interface] Migrate to null safety (flutter-team-arch…

…ive#3497)

[ios_platform_images] Migrate to null safety (flutter-team-archive#3381)

[file_selector_platform_interface] File selector nnbd (flutter-team-a…

…rchive#3509)

Migrating file_selector_interface to null-safety

[file_selector_platform_interface] File selector nnbd (flutter-team-a…

…rchive#3509)

Migrating file_selector_interface to null-safety

Migrate path_provider to null safety. (flutter-team-archive#3460)