OZG-7378 take into account that username can be empty.

...a/de/ozgcloud/archive/bescheid/ExportBescheidService.java

...ain/java/de/ozgcloud/archive/common/user/UserService.java

...ud/archive/historie/AssignedUserChangeHistoryBuilder.java

...n/java/de/ozgcloud/archive/historie/UserProfileCache.java

...de/ozgcloud/archive/kommentar/ExportKommentarService.java

.../de/ozgcloud/archive/postfach/ExportNachrichtService.java

.../ozgcloud/archive/bescheid/ExportBescheidServiceTest.java

.../ozgcloud/archive/common/user/UserProfileTestFactory.java

...java/de/ozgcloud/archive/common/user/UserServiceTest.java

...a/de/ozgcloud/archive/export/ExportGrpcServiceITCase.java

.../java/de/ozgcloud/archive/export/ExportServiceITCase.java

...rchive/historie/AssignedUserChangeHistoryBuilderTest.java

...va/de/ozgcloud/archive/historie/UserProfileCacheTest.java

...oud/archive/historie/VorgangChangeHistoryServiceTest.java

...zgcloud/archive/kommentar/ExportKommentarServiceTest.java

...ozgcloud/archive/postfach/ExportNachrichtServiceTest.java