fix: prevent the frontend from crashing on invalid indexes in results

[s-h-a-d-o-w]

When the index type specified in a result can't be matched to the config classes of a given database, it currently crashes to frontend because of the attempt to make a call on a NoneType.

This makes it so that the empty config is used in such cases but also prints an error to the console, so that people can ideally fix the problematic index config.

Fixes #512

[sre-ci-robot]

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This pull-request has been approved by: alwayslove2013, s-h-a-d-o-w
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[alwayslove2013]

@s-h-a-d-o-w Nice! This will greatly improve the debugging capabilities of vdbbench, making it easier for users to identify problems when extending support to other clients. That's fantastic!

Currently, there are some format and lint issues in this PR. Please try to address those.

pip install ruff --upgrade
pip install black --upgrade
make format
make lint

[s-h-a-d-o-w]

Sorry about that! Passes on my machine now.