docviewer build task fails because Webpack plugins is not defined in config

When running npm run build after running npm install, the build will fail because config.plugins is never defined as an array. The fix is just to check if it's undefined and make it an array if so.

Here is the error:

Running "concurrent:build" (concurrent) task
    Running "hexo:generate" (hexo) task
    Running "docviewer:build" (docviewer) task

    /home/james/Code/SitePen/dojo/dojo.io/docviewer/webpack.config.ts:31
    config.plugins!.push(new optimize.UglifyJsPlugin());
    ^
    TypeError: Cannot read property 'push' of undefined

screenshot_20181026_155711

dojo / dojo.io

docviewer build task fails because Webpack plugins is not defined in config #480