Not having targetScheme for tvos breaks the app

[mihaiblaga89]

Describe the bug
Not having

{
    "platforms": {
          "tvos": {
                  "schemeTarget": "RNVApp-tvOS"
              }
    }
}

will break the xcodebuild with error code 70, means that the target for the build was not found. We might want to default to it if not specified

info Found Xcode workspace "RNVApp.xcworkspace"
info Building (using "xcodebuild -workspace RNVApp.xcworkspace -configuration Debug -scheme RNVApp -destination id=D704E3E7-3468-476F-84C4-FA02C3C2CAD6")

info 💡 Tip: Make sure that you have set up your development environment correctly, by running npx react-native doctor. To read more about doctor command visit: https://github.com/react-native-community/cli/blob/main/packages/cli-doctor/README.md#doctor

UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "Error: Failed to build iOS project. "xcodebuild" exited with error code '70'. To debug build logs further, consider building your app with Xcode.app, by opening 'RNVApp.xcworkspace'.".

To Reproduce
Steps to reproduce the behavior:

1. remove schemeTarget from template-starter and run it on tvos
2. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots or copy&paste
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Desktop (please complete the following information):

• OS: [e.g. osx, windows 7, ubuntu 16.04]
• Node Version [e.g. v10.16.3]
• RNV Version [e.g. 0.25.0]

Additional context
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