🐛 [BUG] Choropleth 在没有区县的城市时，会直接忽略该城市

### 📷 Step to reproduce
[复现链接](https://codesandbox.io/s/nervous-merkle-fh5nqv?file=/index.ts)

广东省中山市和东莞市是没有区县的，因为我设置了没有数据时的兜底颜色
```
scale: {
type: "quantize",
unknown: "rgba(193, 201, 213, 0.28)"
}
```
也就是图中的灰色，但是很明显这两个市，被直接忽略了

### 🏞 Expected result
预期是希望这两个市，也应该是灰色，不要被忽略

- **L7Plot Version**: 0.5.6
- **Platform**: Chrome

Start with the linked CodeSandbox reproduction and its /index.ts entry point, then trace how the choropleth handles cities without county-level data. Done means Zhongshan and Dongguan are rendered using the configured fallback gray color instead of being omitted.

Written by the indexing model from the issue text.