fix bug if experimental design is not given

[timosachsenberg]

now treated as unfractionated

Description

We previously decided to fall back to unfractionated but the initial implementation had a bug.
Should be fixed now with this PR

Checklist:

• Make sure that you are listed in the AUTHORS file
• Add relevant changes and new features to the CHANGELOG file
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• Updated or added python bindings for changed or new classes. (Tick if no updates were necessary.)

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If your PR is failing you can check out

• http://cdash.openms.de/index.php?project=OpenMS and look for your PR. If you click in the column that lists the failed tests you will get detailed error messages.

Note:

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[timosachsenberg]

failed test unrelated