minifier-postcss problems with react-virtualized/styles.css

While trying to build an App that uses react-virtualized I run into the following problem:
```
Errors prevented bundling:
While minifying app stylesheet:
packages/minifier-postcss/plugin/minify-css.js:228:40: Cannot read property 'getContentsAsString' of undefined
at packages/minifier-postcss/plugin/minify-css.js:228:40
at Array.map ()
at mergeCss (packages/minifier-postcss/plugin/minify-css.js:227:36)
at CssToolsMinifier.processFilesForBundle (packages/minifier-postcss/plugin/minify-css.js:88:18)
```

Removing the import of react-virtualized/styles.css fixes the problem.

This seems to be the same problem, the standard minified has had: https://github.com/meteor/meteor/issues/10112

Start in packages/minifier-postcss/plugin/minify-css.js at lines 88 and 227-228, then reproduce the failure by bundling an app that imports react-virtualized/styles.css. Trace why mergeCss receives an undefined value and verify that the stylesheet bundles and minifies without the reported getContentsAsString error.

Written by the indexing model from the issue text.