#127 allow nullable year

src/main/java/ru/ulstu/paper/service/PaperService.java

@@ -361,7 +361,7 @@ public class PaperService {
                 referenceDto.getAuthors(),
                 referenceDto.getPublicationTitle(),
                 StringUtils.isEmpty(referenceDto.getPublisher()) ? "" : referenceDto.getPublisher() + ", ",
-                referenceDto.getPublicationYear().toString(),
+                referenceDto.getPublicationYear() != null ? referenceDto.getPublicationYear().toString() : "",
                 referenceDto.getPages(),
                 StringUtils.isEmpty(referenceDto.getJournalOrCollectionTitle()) ? "." : " // " + referenceDto.getJournalOrCollectionTitle() + ".");
     }
@@ -369,7 +369,7 @@ public class PaperService {
     public String getSpringerReference(ReferenceDto referenceDto) {
         return MessageFormat.format("{0} ({1}) {2}.{3} {4}pp {5}",
                 referenceDto.getAuthors(),
-                referenceDto.getPublicationYear().toString(),
+                referenceDto.getPublicationYear() != null ? referenceDto.getPublicationYear().toString() : "",
                 referenceDto.getPublicationTitle(),
                 referenceDto.getReferenceType() == ARTICLE ? " " + referenceDto.getJournalOrCollectionTitle() + "," : "",
                 StringUtils.isEmpty(referenceDto.getPublisher()) ? "" : referenceDto.getPublisher() + ", ",