[BUG] Node null-check predicate aborts with `vertex variable missing property`

[d1zzy126]

Describe the bug
NeuG aborts on a valid null-check predicate over a bound node variable. A simple query like MATCH (a:Node) WHERE a IS NOT NULL RETURN 1 should execute normally, but the reproduced build aborts in variable.cc:274 with vertex variable missing property, then exits with signal 6.

Execution Logs
The commands below reproduce the issue.

from __future__ import annotations

import shutil
import tempfile
from pathlib import Path

from neug import Database


def main() -> None:
    db_path = Path(tempfile.mkdtemp(prefix="neug_bug_005_"))
    print(f"Using database path: {db_path}", flush=True)

    db = Database(str(db_path), mode="w")
    conn = db.connect()

    setup = [
        ("CREATE NODE TABLE Node(id INT64, PRIMARY KEY(id));", "schema"),
        ("CREATE (a:Node {id: 1});", "update"),
    ]

    try:
        for idx, (query, mode) in enumerate(setup, start=1):
            print(f"STEP {idx}: {query}", flush=True)
            conn.execute(query, access_mode=mode)

        control_query = "MATCH (a:Node) RETURN 1"
        print(f"STEP {len(setup) + 1}: {control_query}", flush=True)
        control_rows = list(conn.execute(control_query, access_mode="read"))
        print(f"Control result: {control_rows}", flush=True)

        crash_query = "MATCH (a:Node) WHERE a IS NOT NULL RETURN 1"
        print(f"STEP {len(setup) + 2}: {crash_query}", flush=True)
        print("Expected: query returns one row.", flush=True)
        print(
            "Actual on the reproduced NeuG build: process aborts with "
            "`vertex variable missing property` followed by signal 6.",
            flush=True,
        )
        list(conn.execute(crash_query, access_mode="read"))
    finally:
        try:
            conn.close()
        except Exception:
            pass
        try:
            db.close()
        except Exception:
            pass
        shutil.rmtree(db_path, ignore_errors=True)


if __name__ == "__main__":
    main()

Expected behavior
The query should execute normally, just like the control query without the null-check predicate.

Error Message
Observed output from a reproduced run:

Using database path: /tmp/neug_bug_005_xxxxxxxx
STEP 1: CREATE NODE TABLE Node(id INT64, PRIMARY KEY(id));
STEP 2: CREATE (a:Node {id: 1});
STEP 3: MATCH (a:Node) RETURN 1
Control result: [[1]]
STEP 4: MATCH (a:Node) WHERE a IS NOT NULL RETURN 1
Expected: query returns one row.
Actual on the reproduced NeuG build: process aborts with `vertex variable missing property` followed by signal 6.
F... variable.cc:274] vertex variable missing property: tag {
  id: 0
}
...
E... neug_binding.cc:45] Received signal 6, Remove all filelocks