indexes method generates empty strings for features above/below max Y coordinate

Nobody has claimed this yet.

### Context:
Not sure if this is expected behavior, but when the tile-cover `indexes` method is used on a geojson feature that has some coordinates above or below the max y coordinate limit for mercator maps, it generates a bunch of empty strings in addition to the quadkeys. It's not a big deal to handle this where we're using it, but just wanted to flag it in case this isn't expected behavior.

### Steps to reproduce:

```js
const tileCover = require('@mapbox/tile-cover');

const featureMaxY = {
'type': 'Feature',
'properties': {},
'geometry': {
'type': 'Polygon',
'coordinates': [
[
[-110.5, 85.05],
[-110.25, 85.05],
[-110.25, 85.0511287798065],
[-110.5, 85.0511287798065],
[-110.5, 85.05]
]
]
}
};

const featureOverMaxY = {
'type': 'Feature',
'properties': {},
'geometry': {
'type': 'Polygon',
'coordinates': [
[
[-110.5, 85.05],
[-110.25, 85.05],
[-110.25, 85.0511287798066], // Or anything above this
[-110.5, 85.0511287798066], // Or anything above this
[-110.5, 85.05]
]
]
}
};

const quadkeysAtMax = tileCover.indexes(featureMaxY.geometry, { min_zoom: 12, max_zoom: 12 });
console.log('Total quadkeys: ', quadkeysAtMax.length); // 4
console.log('Quadkeys: ', quadkeysAtMax);
// Quadkeys: [ '001100010110', '001100010111', '001100011000', '001100011001' ]

const quadkeysOverMax = tileCover.indexes(featureOverMaxY.geometry, { min_zoom: 12, max_zoom: 12 });
console.log('Total quadkeys: ', quadkeysOverMax.length); // 8
console.log('Quadkeys: ', quadkeysOverMax);
// Quadkeys: [ '001100010110', '001100010111', '001100011000', '001100011001', '', '', '', '' ]
```

Start by running the provided tile-cover.indexes reproduction at the Mercator maximum Y boundary and compare the two feature geometries. The fix is done when coordinates above the limit no longer produce empty strings, while the valid quadkeys remain unchanged.

Written by the indexing model from the issue text.