fix(eject): guard against empty key before access

[bmuenzenmeyer]

closes #7288

During eject.js, check if ownPackage has optionalDependencies before accessing the object.

monkey patched within https://github.com/bmuenzenmeyer/test-app

$ npm run eject

> test-app@0.1.0 eject C:\src\tmp\test-app
> react-scripts eject

NOTE: Create React App 2+ supports TypeScript, Sass, CSS Modules and more without ejecting: https://reactjs.org/blog/2018/10/01/create-react-app-v2.html

? Are you sure you want to eject? This action is permanent. Yes
Ejecting...

Copying files into C:\src\tmp\test-app
  Adding \config\env.js to the project
  Adding \config\modules.js to the project
  Adding \config\paths.js to the project
  Adding \config\pnpTs.js to the project
  Adding \config\webpack.config.js to the project
  Adding \config\webpackDevServer.config.js to the project
  Adding \config\jest\cssTransform.js to the project
  Adding \config\jest\fileTransform.js to the project
  Adding \scripts\build.js to the project
  Adding \scripts\start.js to the project
  Adding \scripts\test.js to the project

Updating the dependencies
  Removing @bmuenzenmeyer/react-scripts from dependencies
  Adding @babel/core to dependencies
  Adding @svgr/webpack to dependencies
  Adding @typescript-eslint/eslint-plugin to dependencies
  Adding @typescript-eslint/parser to dependencies
  Adding babel-eslint to dependencies
  Adding babel-jest to dependencies
  Adding babel-loader to dependencies
  Adding babel-plugin-named-asset-import to dependencies
  Adding babel-preset-react-app to dependencies
  Adding camelcase to dependencies
  Adding case-sensitive-paths-webpack-plugin to dependencies
  Adding css-loader to dependencies
  Adding dotenv to dependencies
  Adding dotenv-expand to dependencies
  Adding eslint to dependencies
  Adding eslint-config-react-app to dependencies
  Adding eslint-loader to dependencies
  Adding eslint-plugin-flowtype to dependencies
  Adding eslint-plugin-import to dependencies
  Adding eslint-plugin-jsx-a11y to dependencies
  Adding eslint-plugin-react to dependencies
  Adding eslint-plugin-react-hooks to dependencies
  Adding file-loader to dependencies
  Adding fs-extra to dependencies
  Adding html-webpack-plugin to dependencies
  Adding identity-obj-proxy to dependencies
  Adding is-wsl to dependencies
  Adding jest to dependencies
  Adding jest-environment-jsdom-fourteen to dependencies
  Adding jest-resolve to dependencies
  Adding jest-watch-typeahead to dependencies
  Adding mini-css-extract-plugin to dependencies
  Adding optimize-css-assets-webpack-plugin to dependencies
  Adding pnp-webpack-plugin to dependencies
  Adding postcss-flexbugs-fixes to dependencies
  Adding postcss-loader to dependencies
  Adding postcss-normalize to dependencies
  Adding postcss-preset-env to dependencies
  Adding postcss-safe-parser to dependencies
  Adding react-app-polyfill to dependencies
  Adding react-dev-utils to dependencies
  Adding resolve to dependencies
  Adding sass-loader to dependencies
  Adding semver to dependencies
  Adding style-loader to dependencies
  Adding terser-webpack-plugin to dependencies
  Adding ts-pnp to dependencies
  Adding url-loader to dependencies
  Adding webpack to dependencies
  Adding webpack-dev-server to dependencies
  Adding webpack-manifest-plugin to dependencies
  Adding workbox-webpack-plugin to dependencies

Updating the scripts
  Replacing "react-scripts start" with "node scripts/start.js"
  Replacing "react-scripts build" with "node scripts/build.js"
  Replacing "react-scripts test" with "node scripts/test.js"

Configuring package.json
  Adding Jest configuration
  Adding Babel preset
  Adding ESLint configuration

Running npm install...
audited 901969 packages in 34.949s
found 0 vulnerabilities

Ejected successfully!

Staged ejected files for commit.

Please consider sharing why you ejected in this survey:
  http://goo.gl/forms/Bi6CZjk1EqsdelXk1

[petetnt]

LGTM