I have a df that contains snapshots for JIRA ticket status, df contains multiple snapshots for these tickets hence there are some duplications. I want to fill the null values (as long as the id has Null at one point )) in the previous snapshots with the current latest ticket status. 

For instance:

```
snapshot  id  status    date
week1     111  Null     2019-10-10
week2     111  Null     2019-10-17
week3     111  Analysis 2019-10-23

```

I want to change week1 and week2's status to analysis. 

Another scenario is:

```
snapshot  id  status    date
week1     111  Null     2019-10-10
week2     111  Triage   2019-10-17
week3     111  Analysis 2019-10-23

```

week2 is Triage, but the latest week3 is Analysis, df should still be changed to:

```
snapshot  id   status   date 
week1     111  Analysis 2019-10-10
week2     111  Analysis 2019-10-17
week3     111  Analysis 2019-10-23

```