plugin_constants library Null safety

1. printersdk
2. plugin_constants library

printersdk package

1. Libraries
2. bluetooth_device
3. cpcl
4. esc
5. plugin_constants
6. print_cmd
7. printer.pb
8. printer.pbenum
9. printer.pbjson
10. printer.pbserver
11. printer_device
12. printer_models
13. printer_plugin_impl
14. printer_plugin_web
15. printersdk
16. tcp_device
17. tspl
18. tty_device
19. usb_accessory_device
20. usb_device
21. zpl