FIX(kedro-airflow): grouping with fork

[sbrugman]

Description

Fixes a bug in the grouping when multiple groups need to be merged.

Development notes

Discovered after running on new DAGs.

Checklist

• [X] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [ ] Updated the documentation to reflect the code changes
• [ ] Added a description of this change in the relevant RELEASE.md file
• [ ] Added tests to cover my changes

[astrojuanlu]

Tests still failing:

FAILED tests/test_node_grouping.py::test_group_memory_nodes[memory_nodes0-expected_nodes0-expected_dependencies0] - AttributeError: 'NoneType' object has no attribute 'append'

[merelcht]

Hi @sbrugman, do you want to continue working on this PR?

[sbrugman]

Yes, I'll rebase and fix the error.