List-comprehension projection over a `null` element may produce a nested list instead of `null`.

[Silence6666668]

Describe the bug
List-comprehension projection over a null element may produce a nested list instead of null.

In the repro below, Apache AGE evaluates:

[x IN [null] | x + 1]

as:

[[null, 1]]

Instead of:

[null]

This also affects mixed lists such as [1, null, 2], where the null element becomes [null, 1] instead of staying null.

How are you accessing AGE (Command line, driver, etc.)?

• PostgreSQL cypher(...) wrapper through the local Python differential-testing harness
• Reproducible directly in psql inside the Docker container

What data setup do we need to do?
No graph data is required beyond creating an empty graph:

SELECT create_graph('fuzz_graph');

What is the necessary configuration info needed?

• Plain Apache AGE Docker image was enough
• Docker image in local repro: apache/age
• AGE extension version: 1.7.0
• PostgreSQL version: 18.1
• Graph name used in repro: fuzz_graph
• No extra extensions or special configuration were required

What is the command that caused the error?

SELECT * FROM cypher('fuzz_graph', $$
  RETURN [x IN [null] | x + 1] AS v
$$) AS (v agtype);

Returned result on AGE:

[[null, 1]]

Expected behavior
The null element should stay null under the arithmetic projection, so the expected result is:

[null]

Neo4j returns [null], and Memgraph also returns [null].

Environment (please complete the following information):

• Version: Apache AGE 1.7.0
• PostgreSQL: 18.1
• Host OS: Windows
• Architecture: x86_64
• Deployment: Docker

Additional context
The same issue appears in a mixed list:

SELECT * FROM cypher('fuzz_graph', $$
  RETURN [x IN [1,null,2] | x + 1] AS v
$$) AS (v agtype);

Apache AGE returns:

[2, [null, 1], 3]

Expected result:

[2, null, 3]

Neo4j and Memgraph both return [2, null, 3].

Two nearby control cases behave correctly on the same AGE instance:

1. The same list comprehension without arithmetic projection:

SELECT * FROM cypher('fuzz_graph', $$
  RETURN [x IN [1,null,2] | x] AS v
$$) AS (v agtype);

Observed result:

[1, null, 2]

2. The arithmetic projection on a list without null elements:

SELECT * FROM cypher('fuzz_graph', $$
  RETURN [x IN [1,2] | x + 1] AS v
$$) AS (v agtype);

Observed result:

[2, 3]

So the issue appears specifically when the list-comprehension projection expression operates on a null element.