Add test coverage for bundlerUtils getCssExtractPlugin default case

[justin808]

Missing Test Case

The getCssExtractPlugin function in bundlerUtils.js isn't tested for the default (webpack) case when assets_bundler is undefined.

Suggested Test

it('getCssExtractPlugin defaults to mini-css-extract-plugin when bundler undefined', () => {
  mockConfig.assets_bundler = undefined;
  jest.doMock('shakapacker', () => ({ config: mockConfig }));
  const utils = require('../../../config/webpack/bundlerUtils');
  
  const plugin = utils.getCssExtractPlugin();
  expect(plugin.name).toBe('MiniCssExtractPlugin');
});