keyHash method Null safety

1. pdf_crypto
2. pdf_crypto
3. PdfEncryptionRC4
4. keyHash method

PdfEncryptionRC4 class

1. Constructors
2. PdfEncryptionRC4
3. build
4. Properties
5. elapsedStopwatch
6. encryptionKey
7. hashCode
8. length
9. objgen
10. objser
11. params
12. pdfDocument
13. runtimeType
14. Methods
15. debugFill
16. encrypt
17. keyHash
18. noSuchMethod
19. prepare
20. ref
21. setInsertion
22. startStopwatch
23. stopStopwatch
24. toString
25. write
26. writeContent
27. writeDebug
28. Operators
29. operator ==