[iOS] Don't apply empty attributed string for TextInput

[zhongwuzw]

Summary

Issue reported by @Titozzz , TextInput would shrink when we have attributes like lineHeight.
Demonstration can see GIF like below:
https://giphy.com/gifs/KGNs1qIMHF3DIk1EPK

I think the reason is we apply an empty attributed string to UITextField, now if the length is 0, we just nil the attributedText instead.

Changelog

[iOS] [Fixed] - Don't apply empty attributed string for TextInput

Test Plan

Demo code:

import React from 'react';
import { StyleSheet, TextInput, View } from 'react-native';

export default (App = () => {
  return (
    <View style={styles.container}>
      <View
        style={{
          padding: 20,
          backgroundColor: 'purple',
          flexDirection: 'row',
        }}
      >
        <TextInput
          style={{
            backgroundColor: 'cyan',
            flex: 1,
            lineHeight: 24,
            fontSize: 16,
          }}
        />
      </View>
    </View>
  );
});

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  },
});

[facebook-github-bot]

@sammy-SC has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @zhongwuzw in 9b61896.

^{When will my fix make it into a release? | Upcoming Releases}