fix tinkerpop test open graph panic

Author: Pengzna

hugegraph-pd/hg-pd-client/src/main/java/org/apache/hugegraph/pd/client/PDConfig.java

@@ -101,6 +101,10 @@ public String toString() {
     }
 
     public PDConfig setAuthority(String userName, String pwd) {
+        // If userName is null or empty, keep default values for test environment
+        if (StringUtils.isEmpty(userName)) {
+            return this;
+        }
         this.userName = userName;
         String auth = userName + ':' + pwd;
         this.authority = Base64.getEncoder().encodeToString(auth.getBytes(UTF_8));