TextureCube.data ignores clearOnUndefined

The parameter is used e.g. by `resize` to indicate that empty data should be assigned in order to actually change the buffer size. Instead, nothing happens since no data is passed. This results in `resize` not resizing the texture.

Expected usage of `resize`:
```
this._texture.resize(this._resolution, true, true);
```
This does not resize the texture, as the buffers are unaffected.

Workaround for resizing a `TextureCube` - manually setting new buffers:
```
this._texture.resize(this._resolution, true, false);
const size = 4 * this._resolution * this._resolution; // for RGBA tex
this._texture.data({
clearOnUndefined: true,
positiveX: new Uint8Array(size),
negativeX: new Uint8Array(size),
positiveY: new Uint8Array(size),
negativeY: new Uint8Array(size),
positiveZ: new Uint8Array(size),
negativeZ: new Uint8Array(size),
}, 0, false, true);
```

Start by locating the TextureCube.data implementation and its interaction with resize. Reproduce the reported resize call with clearOnUndefined enabled, then verify that the texture buffers change size without manually supplying six new Uint8Array buffers.

Written by the indexing model from the issue text.