`sum(...)` may mishandle `null` inputs.

[Silence6666668]

Describe the bug
sum(...) may mishandle null inputs.

In Apache AGE, sum(null) returns null instead of 0, and sum(x) over a list containing both numbers and null may fail with:

arguments must resolve to a number

On both Neo4j and Memgraph, sum ignores null inputs and returns a numeric result.

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', $$
  RETURN sum(null) AS v
$$) AS (v agtype);

Returned result on AGE:

null

Expected behavior
The result should be:

0

Neo4j returns 0, and Memgraph also returns 0.

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
A second variant shows the same issue more strongly:

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

Apache AGE returns:

ERROR: arguments must resolve to a number

Expected result:

3

Neo4j returns 3, and Memgraph also returns 3.

Other nearby control cases behave as expected on the same AGE instance:

1. Summing a non-null number works:

SELECT * FROM cypher('fuzz_graph', $$
  RETURN sum(1) AS v
$$) AS (v agtype);

Observed result:

1

2. avg(null) still returns null, which is expected:

SELECT * FROM cypher('fuzz_graph', $$
  RETURN avg(null) AS v
$$) AS (v agtype);

Observed result:

null

3. Even the single-null UNWIND case differs from Neo4j and Memgraph:

SELECT * FROM cypher('fuzz_graph', $$
  UNWIND [null] AS x
  RETURN sum(x) AS v
$$) AS (v agtype);

Apache AGE returns:

null

while Neo4j and Memgraph both return:

0

So the issue appears to be specifically in how Apache AGE's sum aggregate handles null-valued inputs.