Nullability Annotations to Java Classes - Model

[ParaskP7]

Parent #18904

This issue is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related Model classes as possible, including fields, arguments, parameters, and return values.

FYI: As a reference, few examples of such classes are NoteBlock.java, UserNoteBlock.java and CommentUserNoteBlock.java.

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### Tasks
- [ ] #19026
- [ ] #19027
- [ ] #19028
- [ ] #19029
- [ ] #19030
- [ ] #19031
- [ ] #19032
- [ ] #19033
- [ ] #19034
- [ ] #19035
- [ ] #19036
- [ ] #19037
- [ ] #19038
- [ ] #19039
- [ ] #19040

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[peril-wordpress-mobile[bot]]

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