library: fix bug in radosgw_zone.py

library: fix bug in radosgw_zone.py

If for some reason `get_zonegroup()` returns a failure, we must handle
and make the module exit properly instead of failing with the following
python trace:

```
Traceback (most recent call last):
  File "./AnsiballZ_radosgw_zone.py", line 247, in <module>
    _ansiballz_main()
  File "./AnsiballZ_radosgw_zone.py", line 234, in _ansiballz_main
    exitcode = debug(sys.argv[1], zipped_mod, ANSIBALLZ_PARAMS)
  File "./AnsiballZ_radosgw_zone.py", line 202, in debug
    runpy.run_module(mod_name='ansible.modules.radosgw_zone', init_globals=None, run_name='__main__', alter_sys=True)
  File "/usr/lib64/python3.6/runpy.py", line 205, in run_module
    return _run_module_code(code, init_globals, run_name, mod_spec)
  File "/usr/lib64/python3.6/runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "/usr/lib64/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1610728441.41-685133-218973990589597/debug_dir/ansible/modules/radosgw_zone.py", line 467, in <module>
    main()
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1610728441.41-685133-218973990589597/debug_dir/ansible/modules/radosgw_zone.py", line 463, in main
    run_module()
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1610728441.41-685133-218973990589597/debug_dir/ansible/modules/radosgw_zone.py", line 425, in run_module
    zonegroup = json.loads(_out)
  File "/usr/lib64/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>