Port Python test to Elixir: `21-empty-selector-tests`

[hulkoba]

Overview

This work is done according to "Milestone 3. Convert Python test suite to eunit or exunit" as stated in the Scope of Work with Sovereign Tech Agency Resilience Program.

This PR includes:

• Port 21_empty_selector_tests test to Elixir
• Remove the corresponding Python test

Testing recommendations

Run both:

make elixir and make elixir-search

or

make elixir tests=test/elixir/test/mango/21_empty_selector_test.exs and make elixir-search tests=test/elixir/test/mango/21_empty_selector_test.exs

Related Issues or Pull Requests

Checklist

• [ ] Code is written and works correctly
• [ ] Changes are covered by tests
• [ ] Any new configurable parameters are documented in rel/overlay/etc/default.ini
• [ ] Documentation changes were made in the src/docs folder
• [ ] Documentation changes were backported (separated PR) to affected branches