clearProto3Optional method Null safety

1. spine_client
2. descriptor.pb
3. FieldDescriptorProto
4. clearProto3Optional method

FieldDescriptorProto class

1. Constructors
2. FieldDescriptorProto
3. fromBuffer
4. fromJson
5. Properties
6. defaultValue
7. eventPlugin
8. extendee
9. hashCode
10. info_
11. isFrozen
12. jsonName
13. label
14. name
15. number
16. oneofIndex
17. options
18. proto3Optional
19. runtimeType
20. type
21. typeName
22. unknownFields
23. Methods
24. $_ensure
25. $_get
26. $_getB
27. $_getBF
28. $_getI
29. $_getI64
30. $_getIZ
31. $_getList
32. $_getMap
33. $_getN
34. $_getS
35. $_getSZ
36. $_has
37. $_setBool
38. $_setBytes
39. $_setDouble
40. $_setFloat
41. $_setInt64
42. $_setSignedInt32
43. $_setString
44. $_setUnsignedInt32
45. $_whichOneof
46. addExtension
47. check
48. clear
49. clearDefaultValue
50. clearExtendee
51. clearExtension
52. clearField
53. clearJsonName
54. clearLabel
55. clearName
56. clearNumber
57. clearOneofIndex
58. clearOptions
59. clearProto3Optional
60. clearType
61. clearTypeName
62. clone
63. copyWith
64. createEmptyInstance
65. createMapField
66. createRepeatedField
67. ensureOptions
68. extensionsAreInitialized
69. freeze
70. getDefaultForField
71. getExtension
72. getField
73. getFieldOrNull
74. getTagNumber
75. hasDefaultValue
76. hasExtendee
77. hasExtension
78. hasField
79. hasJsonName
80. hasLabel
81. hasName
82. hasNumber
83. hasOneofIndex
84. hasOptions
85. hasProto3Optional
86. hasRequiredFields
87. hasType
88. hasTypeName
89. isInitialized
90. mergeFromBuffer
91. mergeFromCodedBufferReader
92. mergeFromJson
93. mergeFromJsonMap
94. mergeFromMessage
95. mergeFromProto3Json
96. mergeUnknownFields
97. noSuchMethod
98. setExtension
99. setField
100. toBuilder
101. toDebugString
102. toProto3Json
103. toString
104. writeToBuffer
105. writeToCodedBufferWriter
106. writeToJson
107. writeToJsonMap
108. Operators
109. operator ==
110. Static methods
111. create
112. createRepeated
113. getDefault