[BUGFIX] addModule after/before not working

The cleaned up addModule() method no longer adds the module after a
module, when the target module does not exist.

Expected behaviour: module added to the list.

Observed behaviour: module not added at all.

Change-Id: Ie0d1d0c98f9c7dde81636a40eb18a3a7816392f6
Resolves: #55608
Related: #55122
Releases: 6.2
Reviewed-on: https://review.typo3.org/27284
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Markus Klein
Tested-by: Markus Klein