[internal] Fix access to missing finalizers, always add default finalizer

[astef]

Description

Fixes convert_bd_names_to_selector.py to handle LVGs without finalizers.

Link to backport to v0.4.

Why do we need it, and what problem does it solve?

Client upgrading from <=0.2.3 to >=0.4.0 may see errors in convert_bd_names_to_selector.py related to LVG finalizers being empty. After the current change, script will handle this case properly and add the default finalizer, if it doesn't exist.

What is the expected result?

• clients with errors will update to current fix and see errors are gone
• LVGs without the default finalizer will have one after the migration

Checklist

• Changes were tested in the Kubernetes cluster manually.