Fix error paring null JSON - Issue7941

Closes #7941

Treat a null in JSON, when reading the config of a container, as if the
property was never included.  W/o this fix the null would be saved in the
property as a string with a value of "null".

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8273 from duglin/Issue7941

Fix error paring null JSON - Issue7941