Specifying a `--json-filter` output schema without `-ojson` crashes the cli

[4censord]

This works as expected:

$ cloudscale -ojson \
    custom-image list \
    --json '[].{"uuid": uuid, "name": name}'
[
    {
        "name": "image",
        "uuid": "[...}"
    }, ...
]

while this:

$ cloudscale \
    custom-image list \
    --json '[].{"uuid": uuid, "name": name}'
'NoneType' object has no attribute 'items'

crashes with an error

Nothing critical, but catching this and exiting with an error message would be nice.