[BUGFIX] Fix tests throwing errors in MapperTest

Due to incomplete mocking of dependencies the tests for class Mapper
throw Errors after merging #45622.

Fixes: #45682
Releases: 6.0, 6.1
Change-Id: Ibd285acda2c85fdc3e441866f6429ccb1e130809
Reviewed-on: https://review.typo3.org/18407
Reviewed-by: Daniel Hürtgen
Tested-by: Daniel Hürtgen