Testing a Hive array for IS NULL says not null
I just investigated the same issue: In my case it was due to Hive CBO (Cost based optimizer) erroneously replacing the IS NULL evaluator by a constant operator during query planning.
You can try set hive.cbo.enable=false; to see if it solves the issue.
The bug about structs in https://issues.apache.org/jira/browse/HIVE-21778 looks very similar. Haven't tested it yet, however the patch (preventing Calcite type nullability optimization to apply) look like it should solve this bug too.