For Fn::Join, replace None items with an empty string.

Currently, attempts to join lists with None items results in an error.

This change is required for the next attempt to fix bug: #1184794

Change-Id: Ib599eeb21b7325ff797768100f1556a424a8bde9
(cherry picked from commit 85b363342c0d3a25155c500b42810f9ef9d82a22)