Bugfix for axis labels

The PlotHelper does not set empty axis labels when passing '' as a label: the following, for example,

helpers:
  set_labels:
    x: ''

is simply ignored, since the label is evaluated to False and thus skipped in the set_label function. Instead, the PlotHelper needs to check against None.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @blsqr