Fix unused style fragment shorthand

[marneborn]

Components with shorthand Fragments (<></>) weren't being traversed in no-unused-styles.

Using these styles:

const styles = StyleSheet.create({
  used: {},
  unused: {},
});

const CorrectlyFails1 = () => {
  return (<Text style={styles.used}>Hello</Text>);
}

const CorrectlyFails1 = () => {
  return (<React.Fragment><Text style={styles.used}>Hello</Text></React.Fragment>);
}

const IncorrectlyPasses = () => {
  return (<><Text style={styles.used}>Hello</Text></>);
}

[Intellicode]

thanks @marneborn!