allow for null values in yaml inventory (#26765)

* allow for null values in yaml inventory

* 3 dad3 dad3 dad3 dad3 dad3 dad3 dad3 dad

lib/ansible/plugins/inventory/yaml.py

@@ -126,16 +126,16 @@ class InventoryModule(BaseFileInventoryPlugin):
                     if section in group_data and isinstance(group_data[section], string_types):
                         group_data[section] = {group_data[section]: None}
 
-                if 'vars' in group_data:
+                if group_data.get('vars', False):
                     for var in group_data['vars']:
                         self.inventory.set_variable(group, var, group_data['vars'][var])
 
-                if 'children' in group_data:
+                if group_data.get('children', False):
                     for subgroup in group_data['children']:
                         self._parse_group(subgroup, group_data['children'][subgroup])
                         self.inventory.add_child(group, subgroup)
 
-                if 'hosts' in group_data:
+                if group_data.get('hosts', False):
                     for host_pattern in group_data['hosts']:
                         hosts, port = self._parse_host(host_pattern)
                         self.populate_host_vars(hosts, group_data['hosts'][host_pattern] or {}, group, port)