Avoid NullPointerException if peer certificates array is null

Treat null array the same as empty array

src/java/org/jivesoftware/openfire/sasl/ExternalClientSaslServer.java

@@ -11,6 +11,7 @@ import javax.security.sasl.Sasl;
 import javax.security.sasl.SaslException;
 import javax.security.sasl.SaslServer;
 import java.nio.charset.StandardCharsets;
+import java.security.cert.Certificate;
 import java.security.KeyStore;
 import java.security.cert.X509Certificate;
 import java.util.ArrayList;
@@ -57,14 +58,15 @@ public class ExternalClientSaslServer implements SaslServer
         complete = true;
 
         final Connection connection = session.getConnection();
-        if ( connection.getPeerCertificates().length < 1 )
+        Certificate[] peerCertificates = connection.getPeerCertificates();
+        if ( peerCertificates == null || peerCertificates.length < 1 )
         {
             throw new SaslException( "No peer certificates." );
         }
 
         final KeyStore keyStore = connection.getConfiguration().getIdentityStore().getStore();
         final KeyStore trustStore = connection.getConfiguration().getTrustStore().getStore();
-        final X509Certificate trusted = CertificateManager.getEndEntityCertificate( connection.getPeerCertificates(), keyStore, trustStore );
+        final X509Certificate trusted = CertificateManager.getEndEntityCertificate( peerCertificates, keyStore, trustStore );
         if ( trusted == null )
         {
             throw new SaslException( "Certificate chain of peer is not trusted." );