[Fix](Authenticator)defaultAuthenticator not init

1： The `defaultAuthenticator` is not initialized. When a configured authentication plugin implements the `canDea`l method and returns `false`, the `defaultAuthenticator` will be used, resulting in a login failure. The client exception message is `RROR 2013 (HY000): Lost connection to MySQL server at 'reading authorization packet', system error: 2`. This issue only affects users who use non-built-in authentication plugins that implement `canDeal` and return false under certain circumstances.

apache#40113 introduced this issue

fe/fe-core/src/main/java/org/apache/doris/mysql/authenticate/AuthenticatorManager.java

@@ -44,7 +44,7 @@ public class AuthenticatorManager {
 
     public AuthenticatorManager(String type) {
         LOG.info("Authenticate type: {}", type);
-
+        defaultAuthenticator = new DefaultAuthenticator();
         if (authTypeAuthenticator == null) {
             synchronized (AuthenticatorManager.class) {
                 if (authTypeAuthenticator == null) {