refactor: make categories optional in core config

[BioPhoton]

Closes #407

I aimed for handling the optional categories only in the cli layer and maintained the existing data structure fully in reports. I just hide the sections for empty categories.

Follow-up by @Tlacenka:

• I added tests for scoring and sorting reports.
• I allowed passing both undefined and [] by configuring categories schema to allow empty array.
• I reduced minimal mocks.
• I typed categories as a non-optional property as soon as in the only plugins middleware.

[matejchalk]

We could also consider having some prompt to create categories in place of the missing category sections in report.md or stdout summary 🤔 But that's a different discussion and out of scope of this PR, I believe.

[Tlacenka]

We could also consider having some prompt to create categories in place of the missing category sections in report.md or stdout summary 🤔

@matejchalk
While we log a suggestion, that could get lost in the history. So I agree that having that prompt in the report itself would make sure that people know this option exists and where to find more info about it. Can you create an issue for this, please?

[matejchalk]

While we log a suggestion, that could get lost in the history. So I agree that having that prompt in the report itself would make sure that people know this option exists and where to find more info about it. Can you create an issue for this, please?

Done ✔️

• Add hint to create categories in CLI reports #445

[BioPhoton]

You could update the snippets in 'docs/custom-plugind.md'