fix: postcss-plugin returns now a consistent void return value

Author: jantimon

lib/postcss-plugin.js

@@ -189,11 +189,13 @@ module.exports = postcss.plugin('iconfont-webpack', config => function (root, re
       // Prefix the fontname with a letter as fonts with a leading number are not allowed
       fontName = config.fontNamePrefix + String.fromCharCode(fontName.charCodeAt(0) + 20) + fontName.substr(1);
       // Update the css
-      return Promise.all([
+      const processCssPromise = Promise.all([
         // add the font faces
         addFontDeclaration(fontName, root, config.enforcedSvgHeight, svgPaths),
         // replace the `font-icon` occurences
         replaceIconFontDeclarations(fontName, root, svgPaths)
       ]);
+      // Return an empty promise
+      return processCssPromise.then(function () { });
     });
 });