fix(server): ensure graph clear API does not clear meta table

Author: LYD031106

hugegraph-server/hugegraph-mysql/src/main/java/org/apache/hugegraph/backend/store/mysql/MysqlStore.java

@@ -223,6 +223,7 @@ public void truncate() {
         this.checkOpened();
 
         this.truncateTables();
+        this.init();
         LOG.debug("Store truncated: {}", this.store);
     }