PrintCmd constructor Null safety

1. printersdk
2. print_cmd
3. PrintCmd
4. PrintCmd constructor

PrintCmd class

1. Constructors
2. PrintCmd
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addByte
8. addBytes
9. addString
10. clear
11. noSuchMethod
12. rangeLimit
13. setBlackColorDensity
14. setPrintColor
15. setPrinterDoubleColorMode
16. setRedColorDensity
17. toBytes
18. toString
19. Operators
20. operator ==
21. Constants
22. printColorBlack
23. printColorRed