items property Null safety

1. conduit_open_api
2. open_api_v2
3. APIHeader
4. items property

APIHeader class

1. Constructors
2. APIHeader
3. Properties
4. castMap
5. collectionFormat
6. defaultValue
7. description
8. enumerated
9. exclusiveMaximum
10. exclusiveMinimum
11. extensions
12. format
13. hashCode
14. items
15. maximum
16. maxItems
17. maxLength
18. minimum
19. minItems
20. minLength
21. multipleOf
22. pattern
23. referenceURI
24. representation
25. runtimeType
26. type
27. uniqueItems
28. Methods
29. decode
30. encode
31. noSuchMethod
32. toString
33. Operators
34. operator ==