[cleaning] avoiding NPEs handling null author PIDs

dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/CleaningFunctions.java

@@ -58,9 +58,9 @@ public class CleaningFunctions {
 				}
 			}
 			if (Objects.nonNull(r.getAuthor())) {
-				r.getAuthor().forEach(a -> {
+				r.getAuthor().stream().filter(Objects::nonNull).forEach(a -> {
 					if (Objects.nonNull(a.getPid())) {
-						a.getPid().forEach(p -> {
+						a.getPid().stream().filter(Objects::nonNull).forEach(p -> {
 							fixVocabName(p.getQualifier(), ModelConstants.DNET_PID_TYPES);
 						});
 					}
@@ -218,6 +218,7 @@ public class CleaningFunctions {
 								a
 									.getPid()
 									.stream()
+									.filter(Objects::nonNull)
 									.filter(p -> Objects.nonNull(p.getQualifier()))
 									.filter(p -> StringUtils.isNotBlank(p.getValue()))
 									.map(p -> {