Fix possible null pointer in acumulator toString

Author: Michael Armbrust <michael@databricks.com>

Closes #1204 from marmbrus/nullPointerToString and squashes the following commits:

35b5fce [Michael Armbrust] Fix possible null pointer in acumulator toString

(cherry picked from commit 2714968e1b40221739c5dfba7ca4c0c06953dbe2)
Signed-off-by: Reynold Xin <rxin@apache.org>