NIMMessageAttachment constructor Null safety

1. nim_core
2. nim_core
3. NIMMessageAttachment
4. NIMMessageAttachment constructor

NIMMessageAttachment class

1. Constructors
2. NIMMessageAttachment
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toMap
9. toString
10. Operators
11. operator ==