provide {} value for required&flattened parameter

[changlong-liu]

for this required property: https://github.com/Azure/azure-rest-api-specs/blob/master/specification/migrate/resource-manager/Microsoft.Migrate/stable/2019-10-01/migrate.json#L264

generated code don't set default value {}

Expected:

    group = {}
    group['e_tag'] = e_tag
    group['property'] = {}          # need this line
    return client.create(resource_group_name=resource_group_name,
                         project_name=project_name,
                         group_name=group_name,
                         group=group)

[changlong-liu]

merged