Empty `UNWIND` followed by `MERGE` and a null-filtered `WITH` causes a PostgreSQL backend segmentation fault

Docker image `apache/age:release_PG18_1.7.0

**Reproduction**

```sql
LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_issue_case8_reduced');

SELECT *
FROM ag_catalog.cypher('age_issue_case8_reduced', $$
UNWIND [] AS x
MERGE (n)
WITH x, head(range(9, 23)) AS y
WHERE y IS NULL
RETURN {x: x, y: y} AS row
$$) AS (row agtype);
```

**Expected**

The empty input produces zero rows. The query completes normally, returns
zero rows, and does not create a vertex or terminate the PostgreSQL backend.

**Actual**

```text
server closed the connection unexpectedly
client backend ... was terminated by signal 11: Segmentation fault

Start by reproducing the segmentation fault with the apache/age:release_PG18_1.7.0 Docker image and the SQL query in the issue, using the LOAD, create_graph, and cypher entry points shown. Trace the empty UNWIND followed by MERGE and null-filtered WITH, then verify that it returns zero rows, creates no vertex, and does not terminate the PostgreSQL backend.

Written by the indexing model from the issue text.