How to cast a value to boolean
Here is a concise way using IFNULL() function

SELECT *
FROM TABLE
ORDER BY IFNULL(order_by,'')='';


or adding the IF() function

SELECT *
FROM TABLE
ORDER BY IF(IFNULL(order_by,'')='',1,0);


UPDATE 2017-03-22 14:43 EDT

ypercubeᵀᴹ has one more suggestion:

SELECT *
FROM TABLE
ORDER BY (order_by &gt; '') DESC;