Push argv0 in the event that arguments_ is empty.
Push argv0 in the event that arguments_ is empty.

Change-Id: I707e39e5c738b1d5a30f6cb282cb04d457cf9833

BUG=none
TEST=run unit tests,gmerge login

Review URL: http://codereview.chromium.org/3526027

chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager