F fix merge key word

Summary

It was possible that the merge_entities function ran into a KeyError. This is now fixed.

Focus

The first commit adds a failing test. The second commit introduces the fix.

Test Environment

unittest

Check List for the Author

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• All automated tests pass
• Reference related issues
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  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
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• Check: Are there specifications? Are they satisfied?

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