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In the filtering expression "WHERE m[?] = 2", our implementation was buggy when either the map, or the subscript, was NULL (and also when the latter was an UNSET_VALUE). Our code ended up dereferencing null objects, yielding bizarre errors when we were lucky, or crashes when we were less lucky - see examples of both in issues #10361, #10399, #10401. The existing test `test_null.py::test_map_subscript_null` reproduced all these bugs sporadically. In this series we improve the test to reproduce the separate bugs separately, and also reproduce additional problems (like the UNSET_VALUE). We then **define** both `m[NULL]` and `NULL[2]` to result in NULL instead of the existing undefined (and buggy, and crashing) behavior. This new definition is consistent with our usual SQL-inspired tradition that NULL "wins" in expressions - e.g., `NULL < 2` is also defined as resulting in NULL. However, this decision differs from Cassandra, where `m[NULL]` is considered an error but `NULL[2]` is allowed. We believe that making `m[NULL]` be a NULL instead of an error is more consistent, and moreover - necessary if we ever want to support more complicate expressions like `m[a]`, where the column `a` can be NULL for some rows and non-NULL for others, and it doesn't make sense to return an "invalid query" error in the middle of the scan. Fixes #10361 Fixes #10399 Fixes #10401 Closes #10420 * github.com:scylladb/scylla: expressions: don't dereference invalid map subscript in filter expressions: fix invalid dereference in map subscript evaluation test/cql-pytest: improve tests for map subscripts and nulls