fix: fix PostCSS warning on build command (#1664)

The warning is produced on `cssnano` call. See below:

> Without `from` option PostCSS could generate wrong source map and will not find Browserslist config. Set it to CSS file path or to `undefined` to prevent this warning.

Reproduction:

```shell
mkdir work
cd work
npx docusaurus-init
cd website
yarn install
yarn run build
```

Environments:

- Node: 12.6.0
- Yarn: 1.17.3

packages/docusaurus-1.x/lib/server/utils.js

@@ -53,6 +53,7 @@ function minifyCss(cssContent) {
     .process(cssContent, {
       preset: 'default',
       zindex: false,
+      from: undefined,
     })
     .then(result => result.css);
 }