Remove undefined value on init cli command (#22045)

Summary:
Fixes `undefined` description message when running `react-native init --help` on an existing React Native project.
Pull Request resolved: https://github.com/facebook/react-native/pull/22045

Differential Revision: D12878956

Pulled By: TheSavior

fbshipit-source-id: ede329ca88a02013a6c2f75f1b762d89eacdb34f

local-cli/cliEntry.js

@@ -60,7 +60,7 @@ function printHelpInformation() {
   let output = [
     '',
     chalk.bold(chalk.cyan(`  react-native ${cmdName} ${this.usage()}`)),
-    `  ${this._description}`,
+    this._description ? `  ${this._description}` : '',
     '',
     ...sourceInformation,
     `  ${chalk.bold('Options:')}`,