Bugfix: More robust handling of labels

[Davknapp]

Special signs, empty spaces, etc could cause an error while processing the labels.
If that happens we suppress the error-message (which lead to a failing wf) and add an empty string to the "labels"-variable instead. That way all following labels can be processed without the wf failing.
Closes #1532

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[lenaploetzke]

Seems to work, only the comments should be improved.