oneOf property Null safety

1. conduit_open_api
2. open_api_v3
3. APISchemaObject
4. oneOf property

APISchemaObject class

1. Constructors
2. APISchemaObject
3. array
4. boolean
5. empty
6. file
7. freeForm
8. integer
9. map
10. number
11. object
12. string
13. Properties
14. additionalPropertyPolicy
15. additionalPropertySchema
16. allOf
17. anyOf
18. castMap
19. defaultValue
20. deprecated
21. description
22. enumerated
23. exclusiveMaximum
24. exclusiveMinimum
25. extensions
26. format
27. hashCode
28. isNullable
29. isReadOnly
30. isRequired
31. isWriteOnly
32. items
33. maximum
34. maxItems
35. maxLength
36. maxProperties
37. minimum
38. minItems
39. minLength
40. minProperties
41. multipleOf
42. not
43. oneOf
44. pattern
45. properties
46. referenceURI
47. runtimeType
48. title
49. type
50. uniqueItems
51. Methods
52. decode
53. encode
54. noSuchMethod
55. toString
56. Operators
57. operator ==