Add missing test assets
Add missing test assets

This patch adds missing assets/empty.html which is needed for one of
the puppeteer tests.

ianchn/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,elisherer/puppeteer,ianchn/puppeteer,a1ph/puppeteer,michaelfward/puppeteer,GoogleChrome/puppeteer,brutalcrozt/puppeteer,GoogleChrome/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,brutalcrozt/puppeteer,michaelfward/puppeteer,a1ph/puppeteer,elisherer/puppeteer,GoogleChrome/puppeteer