Fix retrieval of customer settings

[GoebelL]

Changes in this Pull Request: Retrieval of customer settings:

• null safe
• read customer settings from custom attributes with fallback instance attributes and parameters

Sorry for many rebase commits.

Mandatory checks:

• [X] latest changes were fetched from the upstream branch before creating this PR (branch is up to date)
• [ ] changes were tested manually
• [ ] unit tests were properly implemented
• [ ] communication about interdependent changes has been sent to the team (if applicable)
• [ ] respective Jenkins jobs were referenced in additional context section of this PR

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This change is