fix: config fails to load with --platform flag

[TMisiukiewicz]

Summary:

When dependency does not have platforms property in dependency config, the config command was breaking when passing --platform [platformName] when it was trying to read platforms from such dependencies.

E.g. @react-native-webapis/web-storage contains react-native.config.js file where platforms are not defined, so config was breaking when trying to access it.

Closes #2463

Test Plan:

1. Run node ./path-to-cli/cli/build/bin.js config --platform ios
2. Verify the output was returned properly
3. Run node ./path-to-cli/cli/build/bin.js config --platform android
4. Verify the output was returned properly

Checklist

• [x] Documentation is up to date to reflect these changes.
• [x] Follows commit message convention described in CONTRIBUTING.md

[szymonrybczak]

@micaww mind checking if the solution work? To test this change you can simple change the line inside node_modules/@react-native-community/cli-config/...