Fix trailing whitespaces on 'empty' lines in XDMF output.

Tom Fischerrequested to merge
TomFischer/ogs:fix_whitespaces_only_lines_in_xdmf into master
  1. Feature description was added to the changelog
  2. Tests covering your feature were added? - Usual xdmf-comparisons should run.
  3. Any new feature or behavior change was documented?

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