Handle the case where archive_name isn't present
Handle the case where archive_name isn't present

This CL fixes an issue we're seeing on the Flutter waterfall when zipping a
directory.

R=iannucci@google.com

Review URL: https://codereview.chromium.org/1710933006

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@298889 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build