`Array.prototype.at` polyfill works inconsistent with spec.

The following code works different in node 15 and 16:

```javascript
 const at = require('core-js-pure/actual/array/at');
at({ length: BigInt("-10") }, 0)
```


```
Running node v16.20.2 (npm v8.19.4)
TypeError: Cannot convert a BigInt value to a number
```

```
Running node v15.14.0 (npm v7.7.6)
undefined
```

