bug: accept use with addPlugin only

[gimler]

i have a multi config setup. In one of my configs i only call addPlugin().

const path = require('node:path');
const fileName = process.env.COPY_FILE_NAME || false;
let copyConfig = {};
if (fileName) {
  copyConfig = {
    from: `${fileName}`,
    to: `${path.dirname(path.relative(path.resolve(__dirname, 'picture'), fileName))}/[name].[contenthash:8][ext]`,
  }
} else {
  copyConfig = {
    from: './picture',
    to: '[path][name].[contenthash:8][ext]'
  }

  Encore.cleanupOutputBeforeBuild()
};

const CopyPlugin = require('copy-webpack-plugin');
Encore
...
  .addPlugin(
      new CopyPlugin({
        patterns: [copyConfig],
      })
    )
...

this config only copy files optional only a specific ;)

Actually only use addPlugin() is not supported. I change the code and add test for the scenario.

releated #1140

[gimler]

@weaverryan can you take a look please?

[weaverryan]

Thanks Gordon!