Fix SCons generator in the face of empty files lists.
Fix SCons generator in the face of empty files lists.

When the list of files to copy is generated based on varables, it can
reasonably end up empty. Ubuntu's builds are hitting this.

http://codereview.chromium.org/174020

old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google