v5: implicit-null mapping key dropped when it is the last entry before a document marker

v5: implicit-null mapping key dropped when it is the last entry before a document marker

### What happens

In v5, a mapping key whose value is an implicit null is dropped if it is the last entry of a document that ends with an explicit document marker (`---` or `...`). No error is raised, the key is just missing.

```js
import { loadAll } from 'js-yaml'

loadAll('a:\n---\nx: 1\n')
// v4.3.0 => [ { a: null }, { x: 1 } ]
// v5.2.2 => [ {},          { x: 1 } ]
```

### Expected

`[ { a: null }, { x: 1 } ]`, as in v4. `yaml` (eemeli) also returns `{ a: null }` for this input.

### Scope

Only the combination of implicit null value, last entry in the document, and an explicit terminator triggers it. Everything adjacent is correct, which makes it easy to miss:

| input | v5 result | note |
| --- | --- | --- |
| `m:\n  a:\n  b:\n---\nx: 1\n` | `{m: {a: null}}` | `b` dropped |
| `a:\nb:\n---\nx: 1\n` | `{a: null}` | `b` dropped, nesting irrelevant |
| `m:\n  a:\n  b: 2\n---\nx: 1\n` | correct | last value is not null |
| `m:\n  a:\n  b: null\n---\nx: 1\n` | correct | explicit null is fine |
| `m:\n  - a\n  -\n---\nx: 1\n` | correct | sequences unaffected |
| `m:\n  a:\n  b:\n` | correct | no terminator |
| `m:\n  a:\n  b:\n...\n` | `{m: {a: null}}` | `...` also triggers it |

Not schema dependent: the same result occurs with default options, an explicit `CORE_SCHEMA`, `YAML11_SCHEMA`, and `FAILSAFE_SCHEMA`-derived schemas.

### Versions

Reproduces on 5.0.0, 5.1.0, 5.2.0, 5.2.1 and 5.2.2. Correct on 4.1.1 and 4.3.0. Verified on Node v24.18.0 and Bun 1.3.10.