Modify username column to be NOT NULL with default

Set username column to NOT NULL with a default value.

Author: adela-bytebase

migrations/1.0.0_create_t1.sql

@@ -1,4 +1,4 @@
 CREATE TABLE t0 (
     id SERIAL PRIMARY KEY,
-    username VARCHAR(50) 
+    username VARCHAR(50) NOT NULL DEFAULT ''
 );