Transparent underline changes label to transparent

### Current behaviour

When activeUnderline (or activeOutline) is set to transparent, the label turns transparent as well.

### Expected behaviour

The label should use the theme color.

### Code sample

[Link to snack](https://snack.expo.dev/@p-syche/textinput-transparent-underline) with a repro.

### Your Environment

| software | version
| --------------------- | -------
| ios or android | Android 10
| react-native | 0.66.3
| react-native-paper | 4.10.1
| node | 14.18.1
| npm | 8.1.4
| expo | 43.0.3
| react-native-vector-icons | 9.0.0

### Shoutouts

This issue was discovered by @r6rob and described in a [comment](https://github.com/callstack/react-native-paper/pull/1797#issuecomment-978731341) to the PR introducing custom underline color, by @belgamo .

从链接的 Expo Snack 开始重现问题，然后追踪 React Native Paper 组件对 activeUnderline 和 activeOutline 的处理。验证将任一 prop 设置为 transparent 后 label 仍使用主题颜色，并确认在所报告的 Android 配置上的行为。

由索引模型根据 Issue 内容生成。