fix: When a doris field has default value '' (empty str), the ddl to modify the column type is incorrectly

[potterhe]

Proposed changes

Problem Summary:

CREATE TABLE `modify_column_type_with_default_value2` (
  `id` int NOT NULL,
  `cname` varchar(10) NOT NULL DEFAULT ""
)
UNIQUE KEY(`id`)
DISTRIBUTED BY HASH(`id`) BUCKETS 1
PROPERTIES (
"replication_num" = "1"
);

call api to change cname column: varchar(10) -> varchar(11)

        String columnName = "cname";
        String newColumnType = "varchar(11)";
        FieldSchema field = new FieldSchema(columnName, newColumnType, "");
        schemaChangeManager.modifyColumnDataType(DATABASE, modifyColumnTbls, field);

doris throw exception

org.apache.doris.flink.exception.DorisSchemaChangeException: Failed to schemaChange, response: {"msg":"Error","code":1,"data":"Failed to execute sql: java.sql.SQLException: (conn=1786844) errCode = 2, detailMessage = Can not change default value","count":0}

The actual generated ddl is

ALTER TABLE `test_sc_db`.`modify_column_type_with_default_value2` MODIFY COLUMN `cname` varchar(11)

Expected generated ddl is

ALTER TABLE `test_sc_db`.`modify_column_type_with_default_value2` MODIFY COLUMN `cname` varchar(11) DEFAULT ''

Checklist(Required)

1. Does it affect the original behavior: (Yes)
2. Has unit tests been added: (Yes)
3. Has document been added or modified: (No Need)
4. Does it need to update dependencies: (No)
5. Are there any changes that cannot be rolled back: (No)

Further comments

n/a

[JNSimba]

LGTM