Can the 'final String? status' field be filtered to exclude null values?

Can the 'final String? status' field be filtered to exclude null values?
The 'status' field can have the values 'null', 'inProgress', 'pending', and 'approved'.

Ex>
```
var filter = FilterState();
filter.add(_statusFilterGroup, [Filter.facet('status', !=null )]); // <<<
```

No file or test is named. Start by locating the Filter.facet and FilterState entry points, then inspect the existing facet-filter tests to determine how null status values are represented. Done means supporting the requested status != null filter and covering the behavior with a regression test.

Written by the indexing model from the issue text.