[ZEPPELIN-6022] Skip decryption of credentials.json when file is empty (

#4765)

* Skip decryption when empty

* Use more elegant empty json string check

zeppelin-zengine/src/main/java/org/apache/zeppelin/user/Credentials.java

@@ -137,7 +137,7 @@ private void loadCredentials() throws IOException {
   private void loadFromFile() throws IOException {
     try {
       String json = storage.loadCredentials();
-      if (json != null && encryptor != null) {
+      if (encryptor != null && StringUtils.isNotBlank(json)) {
         json = encryptor.decrypt(json);
       }