Set a valid default value for drawerWidth
Set a valid default value for drawerWidth

If `drawerWidth` is not specified, the entire view is blacked out. This is rather
surprising behavior; I expected `drawerWidth` to be optional, and I spent a lot
of time in confusion trying to figure out why this was happening.

This simple snack demonstrates the problem: https://snack.expo.io/HyxGVNPcqf

Ideally, the drawer would have a reasonable default width regardless of
device size and/or adapting to the width of the component rendered by
`renderNavigationView`, but having a default that's not completely broken
at least lets the developer get started.
kmagiera/react-native-gesture-handler,kmagiera/react-native-gesture-handler,kmagiera/react-native-gesture-handler,kmagiera/react-native-gesture-handler,kmagiera/react-native-gesture-handler