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

**AGE Version:** apache/age master @ cfd3b634 (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):

```sql
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.

Start with agtype_exists_agtype in src/backend/utils/adt/agtype_ops.c around line 1325, then read extract_entity_properties in agtype.c around lines 12432-12434 and reproduce the SQL queries from the issue in PostgreSQL. Done means the agtype-null left operand no longer causes SIGSEGV or a server restart, while the operator returns false or a clean client-side error; verify the sibling operators remain correct.

Written by the indexing model from the issue text.