Merge pull request #2339 from brendandburns/azure

Add null handling for a missing expiresOn field.

Author: k8s-ci-robot

util/src/main/java/io/kubernetes/client/util/authenticators/AzureActiveDirectoryAuthenticator.java

@@ -50,6 +50,9 @@ public String getToken(Map<String, Object> config) {
   @Override
   public boolean isExpired(Map<String, Object> config) {
     String expiresOn = (String) config.get(EXPIRES_ON);
+    if (expiresOn == null || expiresOn.length() == 0) {
+      return true;
+    }
     Date expiry = new Date(Long.parseLong(expiresOn) * 1000);
     return expiry.compareTo(new Date()) <= 0;
   }