Change return type from bool to None for indicator

[matthiasschaub]

Description

Change return type from bool to None for indicator for functions: preprocess, calculate, create_figure

Corresponding issue

Closes #

New or changed dependencies

Checklist

• I have updated my branch to main (e.g. through git rebase main)
• My code follows the style guide and was checked with pre-commit before committing
• I have commented my code
• I have added sufficient unit and integration tests
• I have updated the CHANGELOG.md

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[Gigaszi]

Everthing looks fine