fix(core): correctly store first EPPN in LDAP

- We encountered possible NULL value if user had no EPPNs.

Author: zlamalp

perun-ldapc/src/main/java/cz/metacentrum/perun/ldapc/model/impl/PerunUserImpl.java

@@ -149,6 +149,7 @@ public void addPrincipal(User user, String login) {
 		if (isEppnEpuidLogin(login)) {
 
 			String[] oldEppns = entry.getStringAttributes(PerunAttribute.PerunAttributeNames.ldapAttrEduPersonPrincipalNames);
+			if (oldEppns == null) oldEppns = new String[0];
 			TreeSet<String> uniqueEppns = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
 			uniqueEppns.addAll(Stream.of(oldEppns).collect(Collectors.toSet()));
 
@@ -161,6 +162,7 @@ public void addPrincipal(User user, String login) {
 		}
 
 		String[] oldIdentities = entry.getStringAttributes(PerunAttribute.PerunAttributeNames.ldapAttrUserIdentities);
+		if (oldIdentities == null) oldIdentities = new String[0];
 		TreeSet<String> uniqueIdentities = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
 		uniqueIdentities.addAll(Stream.of(oldIdentities).collect(Collectors.toSet()));