preSign method Null safety

1. pdf_crypto
2. pdf_crypto
3. PdfSign
4. preSign method

PdfSign class

1. Constructors
2. pkcs7
3. pkcs7Rsa
4. rsaSha1
5. Properties
6. certificates
7. contactInfo
8. hashCode
9. hasMDP
10. location
11. name
12. permissions
13. reason
14. runtimeType
15. Methods
16. noSuchMethod
17. preSign
18. sign
19. toString
20. Operators
21. operator ==
22. Static methods
23. pemCertificate
24. pemPrivateKey