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>
builder: Fix CMD to inject /bin/sh -c when provided with a non-json value.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8284 from erikh/fix_cmd_again

builder: Fix CMD to inject /bin/sh -c when provided with a non-json value
Merge pull request #8273 from duglin/Issue7941

Fix error paring null JSON - Issue7941
Merge https://github.com/docker/docker into add-index-address-override