Text Image throw error if cluster dont have Text Image

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MapboxGl Version 1.8.1
If `text-field` have default a image but for cluster not, it throws an error:

> zone.js:192 Uncaught TypeError: Cannot read property '0' of undefined
at e.set (mapbox-gl.js:29)
at vi.push../src/scripts/mapbox-gl.js.vi.draw (mapbox-gl.js:33)
at oo (mapbox-gl.js:33)
at io (mapbox-gl.js:33)
at Object.symbol (mapbox-gl.js:33)
at po.push../src/scripts/mapbox-gl.js.po.renderLayer (mapbox-gl.js:33)
at po.push../src/scripts/mapbox-gl.js.po.render (mapbox-gl.js:33)
at r._render (mapbox-gl.js:33)
at mapbox-gl.js:33
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)

This error comes at time the the cluster is shown.
this doesnt happend if boath have a image or boath have no one.

```
"text-field": [
"case",
[
"has",
"point_count"
],
[
"format",
"TEST"
],
[
"format",
[
"image",
"/assets/images/icons/gas/segment_extension.png"
]
]
]
```

The report identifies the text-field expression and the cluster rendering path (`vi.draw`, `po.renderLayer`, and `po.render`) in the bundled `mapbox-gl.js`; start by reproducing the failure with the provided expression in Mapbox GL JS 1.8.1. Done means clustered features without an image render successfully when non-cluster features have one, with coverage for both mixed and matching image cases.

Written by the indexing model from the issue text.