Allow InventoryScript JSON with childgroups only

and without hosts and vars

Without this patch, the simplified syntax is triggered when a group
is defined like this:

    "platforms": {
        "children": [
            "cloudstack"
        ]
    }

Which results in a group 'platforms' with 1 host 'platforms'.

more details in https://github.com/ansible/ansible/issues/13655

lib/ansible/inventory/script.py

@@ -103,7 +103,7 @@ class InventoryScript:
             if not isinstance(data, dict):
                 data = {'hosts': data}
             # is not those subkeys, then simplified syntax, host with vars
-            elif not any(k in data for k in ('hosts','vars')):
+            elif not any(k in data for k in ('hosts','vars','children')):
                 data = {'hosts': [group_name], 'vars': data}
 
             if 'hosts' in data: