OZG-6735 impl PR comments

...ain/java/de/ozgcloud/eingang/fim/AntragstellerMapper.java

...rc/main/java/de/ozgcloud/eingang/fim/FimBasedAdapter.java

...er/src/main/java/de/ozgcloud/eingang/fim/FimDataUtil.java

.../main/java/de/ozgcloud/eingang/fim/FimDocumentReader.java

...c/main/java/de/ozgcloud/eingang/fim/FimSchemeAdapter.java

...ter/src/main/java/de/ozgcloud/eingang/fim/FimService.java

...r/src/main/java/de/ozgcloud/eingang/fim/HeaderMapper.java

...va/de/ozgcloud/eingang/fim/common/xml/DocumentHelper.java

...c/test/java/de/ozgcloud/eingang/fim/HeaderMapperTest.java