avoid NPEs

src/main/java/eu/dnetlib/dhp/schema/oaf/StructuredProperty.java

@@ -33,7 +33,7 @@ public class StructuredProperty implements Serializable {
 	public String toComparableString() {
 		return Stream
 			.of(
-				getQualifier().toComparableString(),
+				Optional.ofNullable(getQualifier()).map(q -> q.toComparableString()).orElse(""),
 				Optional.ofNullable(getValue()).map(String::toLowerCase).orElse(""))
 			.filter(StringUtils::isNotBlank)
 			.collect(Collectors.joining("||"));