[CYPHER] tail(null) returns [] instead of null

**Test:** `tailWithNullReturnsNull` (line 168)

**Cypher query to execute:**
```cypher
RETURN tail(null) AS result
```

**Expected behavior:**
```
tail(null) should return null
```

**ArcadeDB result:**
```json
{"result": []}
```

**Neo4j result:**
```json
{"result": null}
```

According to the documentation, `tail(null)` should return `null`, not an empty list.