[BUGFIX] Set missing markers to empty string

Change-Id: I3892f88bdd094b390a22fe94d9b089bd9b8aef71
Resolves: #54112
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/27102
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn