Don't show [] on single err inspect

$ docker inspect foofoo
wil produce:

Error: No such image or container: foofoo
[]

Notice the trailing [].  In cases where there is a single ID/name provided
this extra [] is just noise and makes no sense, it looks weird  :-)

This PR removes the [] only if there is no JSON to be shown at all.

Note that if there are a list of IDs and at least one of them doesn't
result in an error then things work as they do today.

Signed-off-by: Doug Davis <dug@us.ibm.com>