In Oracle, the EMPTY_CLOB function creates an empty CLOB locator i.e. an empty (with 0 length), but non-NULL CLOB value.
In MariaDB, you can use '' constant (empty string).
Oracle:
-- A sample table CREATE TABLE t1 ( c1 CLOB DEFAULT EMPTY_CLOB() );
MariaDB:
-- A sample table CREATE TABLE t1 ( c1 LONGTEXT DEFAULT '' );
For more information, see Oracle to MariaDB Migration.