Double nested IEF can't be submitted if it's parent doesn't have required fields populated

Steps to reproduce:

- add $settings['extension_discovery_scan_tests'] = TRUE; to settings.php
- enable inline_entity_form_test
- go to ief_test_nested1 "Manage fields" page and make "ief_test_nested1" field non required
- go to ief_test_nested2 "Manage fields" page and make "ief_test_nested2" field non required
- navigate to /node/add/ief_test_nested1
- Fill "Title" for node 3 (3rd level)
- Click "Create node 3"
- Expected result: Node 3 is created
- Actual result: Node is not saved and message is displayed: "This value should not be null"

Workaround

- Fill "Title" for node 2 (2nd level)
- Click "Create node 3"
- Node is created. Strangeness....

Might be related to #2618502: IEF Widgets are completely broken. Patch in #2626970: Add testcase for nested inline entity form includes a test that can easily be tweaked to test this. When this is fixed remove workaround that we added to the test in that issue (see comment reference in code).