[Cleaning] trying to avoid NPEs

dhp-workflows/dhp-graph-mapper/src/main/java/eu/dnetlib/dhp/oa/graph/clean/CleaningFunctions.java

@@ -59,12 +59,16 @@ public class CleaningFunctions {
 				}
 			}
 			if (Objects.nonNull(r.getAuthor())) {
-				r.getAuthor().forEach(a -> {
-					if (Objects.nonNull(a.getPid())) {
-						a.getPid().forEach(p -> {
-							fixVocabName(p.getQualifier(), ModelConstants.DNET_PID_TYPES);
-						});
-					}
+				r.getAuthor()
+						.stream()
+						.filter(Objects::nonNull)
+						.forEach(a -> {
+							if (Objects.nonNull(a.getPid())) {
+								a.getPid()
+									.stream()
+									.filter(Objects::nonNull)
+									.forEach(p -> fixVocabName(p.getQualifier(), ModelConstants.DNET_PID_TYPES));
+							}
 				});
 			}
 			if (value instanceof Publication) {