3505 [shared_preferences] Update macOS for NNBD (cla: yes, nnbd, p: shared_preferences, platform-macos)

3515 Remove stray dependency (cla: yes, p: shared_preferences, platform-windows)

3516 [shared_preferences_windows]-Migrate to null safety (cla: yes, nnbd, p: shared_preferences, platform-windows)

3523 [shared_preferences] Migrate platform plugins to null-safety (cla: yes, nnbd, p: shared_preferences, platform-linux, platform-web)

3526 [shared_preferences] Migrate main package to null-safety (cla: yes, p: shared_preferences, platform-android, platform-ios, platform-macos, platform-web)