RadioButton with no text is not centered horizontally in its intrinsic bounds

### Problem description

A `RadioButton` with no text is not centered horizontally in its intrinsic bounds. Instead it looks like it still treats the spacing between the round graphic and the (empty) text as part of its content. Thereby the radio button graphic is shifted to the left.


### Expected behavior

The radio button circle should be centered in its intrinsic bounds when no text is set.

### Environment

- Tabris.js version: 2.0.0
- Device: iPhone SE
- OS: 10.3.2

### Code snippet

```javascript
const {RadioButton, Composite, ui} = require('tabris');

new Composite({background: '#bbbbbb'}).append(
new RadioButton()
).appendTo(ui.contentView);

```