[Nested ViewPager] child ViewPager will not show without a specific height

## Environment
` "react-native-pager-view": "^5.0.12"`
```
System:
OS: macOS 11.2.2
CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Memory: 852.00 MB / 16.00 GB
Shell: 5.8 - /bin/zsh
Binaries:
Node: 12.19.0 - /usr/local/bin/node
Yarn: 1.22.5 - ~/.yarn/bin/yarn
npm: 7.6.0 - /usr/local/bin/npm
Watchman: 4.9.0 - /usr/local/bin/watchman
Managers:
CocoaPods: 1.10.1 - /usr/local/bin/pod
SDKs:
iOS SDK:
Platforms: iOS 14.4, DriverKit 20.2, macOS 11.1, tvOS 14.3, watchOS 7.2
Android SDK:
API Levels: 21, 23, 25, 26, 27, 28, 29
Build Tools: 23.0.1, 26.0.1, 26.0.2, 27.0.3, 28.0.3, 29.0.2
System Images: android-30 | Google APIs Intel x86 Atom
Android NDK: Not Found
IDEs:
Android Studio: 4.1 AI-201.8743.12.41.7042882
Xcode: 12.4/12D4e - /usr/bin/xcodebuild
Languages:
Java: 1.8.0_212 - /usr/bin/javac
npmPackages:
@react-native-community/cli: Not Found
react: 17.0.1 => 17.0.1
react-native: 0.64.0-rc.4 => 0.64.0-rc.4
react-native-macos: Not Found
npmGlobalPackages:
*react-native*: Not Found
```

## Description
```


// black and yellow children will not show without height: 200






```

## Reproducible Demo

Inizia riproducendo l’esempio di ViewPager annidato dell’issue, usando il layout flex esterno e il pager interno senza un’altezza fissa. Esamina il comportamento del layout di ViewPager sulla piattaforma interessata; il lavoro è completato quando le pagine figlie vengono renderizzate senza richiedere un’altezza esplicita.

Scritto dal modello di indicizzazione a partire dal testo della issue.