Handle Cards without Action URLs

Cards without action URLs will cause JSON.parse to receive 'undefined', an
invalid JSON string. The fix here adds a check for the existence of
Action URLs before parsing, short-circuiting the result to null if there
are no action URLs.

BUG=327079
R=rgustafson@chromium.org, skare@chromium.org
TBR=vadimt@chromium.org

Review URL: https://codereview.chromium.org/108143006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239592