fix: Do not display an error message on empty public folder (#6333)

copy-webpack-plugin will generate an error on missing file(s). 

For SPA, error occurs If 'public' folder only contains index.html or 'public' folder is empty.

Author: petergaal91

packages/@vue/cli-service/lib/config/app.js

@@ -244,6 +244,7 @@ module.exports = (api, options) => {
               from: publicDir,
               to: outputDir,
               toType: 'dir',
+              noErrorOnMissing: true,
               globOptions: {
                 ignore: publicCopyIgnore
               }