Use proper AssertJ assertions

[krzyk]

Slight refactoring of AssertJ assertions into shorter, more descriptive and generating better error messages.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes
• All continuous integration builds pass

[marcphilipp]

Thanks! 👍