[1.x] Provide helpful error messaging for insufficient .yml options

[ahinkle]

When creating a blank vapor.yml file and attempting to deploy, you receive type errors:

Warning: Trying to access array offset on value of type null in /laravel-app/vendor/laravel/vapor-cli/src/Manifest.php on line 16

Fatal error: Uncaught TypeError: Laravel\VaporCli\ConsoleVaporClient::validateManifest(): Argument #3 ($manifest) must be of type array, null given, called in /laravel-app/vendor/laravel/vapor-cli/src/Commands/DeployCommand.php on line 91 and defined in /laravel-app/vendor/laravel/vapor-cli/src/ConsoleVaporClient.php:1121

This PR provides a helpful error message when these values are missing. Also, don't create Vapor config files while eating breakfast. 😆