fix(tools-react-native): fix crash on older versions of @react-native-community/cli

[tido64]

Description

Fix crash when using @react-native-community/cli versions older than 7.x

Resolves #3295.

Test plan

Run:

yarn build @rnx-kit/test-app
cd packages/test-app
node -p 'require("../tools-react-native/platform.js").getAvailablePlatforms()'

This should output:

{ android: '', ios: '', windows: 'react-native-windows' }