(bug 44024) ObjectCache changes break XCache 3.x support

Changes in the 1.20.x branch altered the add method to check for the return value of get against
false. Xcache 3.x returns NULL for unset values and so this check was erroneously determining that
all values are set when they were not.

This was causing basic ObjectCache usage to fail.

Change-Id: I0975e5c6a60c2813dc2d02aca84245807b8fb828
(cherry picked from commit 0072e0efeb8591a7761b3d573f8b71ef97fb96ec)