[2.7.1] action-get with --format=json returns 'null' when no parameters are defined for an action

[Dmitrii Shcherbakov]

cat actions.yaml
test: {}

It appears to be that action-get returns 'null' when `--format=json` is used and there are no parameters defined for an action in actions.yaml. An empty dict is returned without `--format=json` which seems inconsistent.

# with an RPDB trace set up in a charm code:
juju run-action <unit> test --wait

(Pdb) subprocess.check_output(['action-get', '--format=json'])
b'null\n'

(Pdb) type(json.loads(subprocess.check_output(['action-get', '--format=json']).decode('utf-8')))
<class 'NoneType'>

(Pdb) subprocess.check_output(['action-get']).decode('utf-8')
'{}\n'

Related discussion:
https://github.com/canonical/operator/pull/132#discussion_r375271096

[Harry Pidcock (hpidcock)]

https://github.com/juju/juju/pull/11499