Mariadb using IF in trigger to fill null value on insert

Trying to use a trigger to a) fill in a new column top_category with values left of a colon, found in category, then b) If there is no colon, then put entire insert value into top_category. I can't get this second trigger structured to create.

https://dbfiddle.uk/jwLle13e

CREATE TABLE qn_txs (
  id SERIAL PRIMARY KEY,
  category VARCHAR(250),
  top_category VARCHAR(250)
);

-- Stage 1 trigger works
--  CREATE TRIGGER `t_add_top_category1` BEFORE INSERT ON `qn_txs` 
--    FOR EACH ROW 
--  SET NEW.`top_category` = LEFT(NEW.category, INSTR(NEW.category, ":") - 1);

DELIMITER //
CREATE TRIGGER `t_add_top_category2` BEFORE INSERT ON `qn_txs` 
     FOR EACH ROW BEGIN
         SET NEW.`top_category` = LEFT(NEW.category, INSTR(NEW.category, ":") - 1)
         -- make sure top_category is not null if there is a value in category
            IF (NEW.`top_category` = '')
             THEN
              SET NEW.`top_category` = NEW.`category`;
           END IF;
END//

DELIMITER;

INSERT INTO qn_txs (category)
VALUES ('I should also be a top cat'), ('bananas:green'), ('baseball:table'), (''), ('Very long string: with spaces');

SELECT * FROM qn_txs;

Here's a table of the desired output after the insert is complete.