Empty clob not getting set to NULL

I'm merging a clob into my oracle database using Groovy. Everything is working fine, except when there is an empty Clob. Instead of making my table column NULL, it's setting it to an empty string.

Clob myClob = CLOB.createTemporary(sql.getConnection(), false, CLOB.DURATION_SESSION);        
while ((row = reader.readNext()) != null) {
    myClob.truncate(0)
    myClob.setString(1,row[17])
        def stmt = /
                    MERGE INTO my_table a
                    USING (
                      SELECT    ${row[0]} id,
                                ${myClob} clob_column
                      FROM dual) e
                    ON (a.id = e.id)
                    WHEN MATCHED THEN
                      UPDATE SET 
                            a.clob_column = e.clob_column
                      /
   def runsql = sql.execute(stmt)

}

I've tried passing "NULL" instead of my Clob value and it works, but I can't seem to figure out how to conditionally pass it to my merge statement when the clob is empty. I can't just set "myClob" to null, because then it fails when it loops and tries to truncate it. I was hoping I could do something like an elvis operator in the closure:

${myClob ?: null} 

or

${if(myClob.length() > 0) then myClob else null}

but neither seem to work.