HADOOP-15291. TestMiniKdc fails on Java 9

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-common-project/hadoop-minikdc/src/test/java/org/apache/hadoop/minikdc/TestMiniKdc.java

@@ -165,7 +165,8 @@ public class TestMiniKdc extends KerberosSecurityTestcase {
       loginContext.logout();
 
     } finally {
-      if (loginContext != null) {
+      if (loginContext != null && loginContext.getSubject() != null
+          && !loginContext.getSubject().getPrincipals().isEmpty()) {
         loginContext.logout();
       }
     }