replaced null value with a more safe empty string as return value

dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/orcidnodoi/oaf/PublicationToOaf.java

@@ -166,7 +166,13 @@ public class PublicationToOaf implements Serializable {
 		// Adding source
 		final String source = getStringValue(rootElement, "sourceName");
 		if (StringUtils.isNotBlank(source)) {
-			publication.setSource(Arrays.asList(mapStringField(source, null)));
+			Field<String> sourceField = mapStringField(source, null);
+			if (sourceField==null) {
+				publication.setSource(null);
+			}
+			else {
+				publication.setSource(Arrays.asList(sourceField));
+			}
 		}
 
 		// Adding titles

dhp-workflows/dhp-doiboost/src/main/java/eu/dnetlib/doiboost/orcidnodoi/util/DumpToActionsUtility.java

@@ -20,7 +20,7 @@ public class DumpToActionsUtility {
 	public static String getStringValue(final JsonObject root, final String key) {
 		if (root.has(key) && !root.get(key).isJsonNull())
 			return root.get(key).getAsString();
-		return null;
+		return new String("");
 	}
 
 	public static List<String> getArrayValues(final JsonObject root, final String key) {