MessageBuilder constructor Null safety

1. nim_core
2. nim_core
3. MessageBuilder
4. MessageBuilder constructor

MessageBuilder class

1. Constructors
2. MessageBuilder
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static methods
12. createAudioMessage
13. createCustomMessage
14. createEmptyMessage
15. createFileMessage
16. createImageMessage
17. createLocationMessage
18. createTextMessage
19. createTipMessage
20. createVideoMessage