Github user khalidhuseynov commented on the issue:
https://github.com/apache/zeppelin/pull/1697
normally when doing this kind of changes, need to:
1. either override `equals` method or make explicitly static `isEmpty`
method that would handle comparison cases. (e.g.
`revision1.equals(Revision.EMPTY)` or `Revision.isEmpty(revision1)`)
2. substitute all instances of that class that previously have been
compared with `null`. for example,
[here](https://github.com/apache/zeppelin/blob/master/zeppelin-server/src/main/java/org/apache/zeppelin/socket/NotebookServer.java#L1400)
an instance of it was compared to `null` to see if checkpoint was successful,
which wouldn't work now and should be changed.
Generally, this kind of changes would be nice to be reviewed, unless that's
hotfix.
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