Incorrect "is not null" result on fusion value

[philrz]

The middle value should not be present in the query output.

$ echo -e '{"a":{"v":1}}\n{"a":null,"b":"x"}\n{"a":{"v":2}}' > data.json &&
  super -j -i fjson -c "where a is not null" data.json 
{"a":{"v":1}}
{"a":null,"b":"x"}
{"a":{"v":2}}

Details

Repro is with super commit d2bf6ef.

The query above is a simplification of the "union" query that's performed as part of the GitHub Archive benchmarks.

FROM 'gha.json'
| UNNEST [...payload.pull_request.assignees, payload.pull_request.assignee]
| WHERE this IS NOT NULL
| AGGREGATE count() BY assignee:=login
| ORDER BY count DESC
| LIMIT 5;

The middle value is excluded as expected if the data is read via the regular JSON reader.

$ super -version
Version: v0.3.0-157-gd2bf6ef03

$ super -j -i json -c "where a is not null" data.json
{"a":{"v":1}}
{"a":{"v":2}}