The ? (exists) operator crashes the server (SIGSEGV) on an agtype-null left operand

[hkjiang26]

AGE Version: apache/age master @ cfd3b63 (2026-08-14), extension 1.8.0, on PostgreSQL 18.6

Installation Method: Docker

API: SQL (psql)

Steps to reproduce

1. On a fresh database, load the extension and run (no data required):

LOAD 'age';
SET search_path = ag_catalog, public;
SELECT 'null'::agtype ? '"a"'::agtype AS r;

The second form SELECT 'null'::agtype ? 'null'::agtype; crashes identically.

Expected behavior

The ? operator tests top-level key/element existence; for a non-entity left operand it should return false or raise a clean client-side error. Its siblings already behave correctly for the same input:

SELECT 'null'::agtype ?| '["a"]'::agtype;   -- ERROR:  scalar object must be a vertex or edge
SELECT 'null'::agtype ?& '["a"]'::agtype;   -- ERROR:  scalar object must be a vertex or edge

Other scalar left operands of ? also work: '1', 'true', '1.5', '"s"', '{}', '[]' all return f without error.

Actual behavior

The client connection drops and the whole PostgreSQL instance restarts (all backends are terminated):

psql: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
psql: error: connection to server was lost

Server log (docker logs):

LOG:  client backend (PID 9073) was terminated by signal 11: Segmentation fault
LOG:  all server processes terminated; reinitializing

Root cause: in agtype_exists_agtype (src/backend/utils/adt/agtype_ops.c:1325), for a scalar-rooted input the code calls extract_entity_properties(agt, false), which returns NULL when the scalar is AGTV_NULL (agtype.c:12432-12434); the result is then passed to agtype_value_to_agtype(), which dereferences the NULL pointer.

Not reachable from Cypher: a Cypher NULL is normalized to SQL NULL and the strict ? function short-circuits before entering the C code (verified). The trigger surface is AGE's public SQL API — any user able to run SQL with the extension loaded can crash the shared instance.