Fix StringListConverter to add empty string in database column if the array is empty.

src/main/java/eu/dnetlib/developers/utils/StringListConverter.java

@@ -12,7 +12,7 @@ public class StringListConverter implements AttributeConverter<List<String>, Str
     @Override
     public String convertToDatabaseColumn(List<String> attribute) {
         if (attribute == null || attribute.isEmpty()) {
-            return null;
+            return "";
         }
         return String.join(ARRAY_DELIMITER, attribute);
     }