Concatenate date and varchar
The CONCAT() function returns NULL when one of the arguments is null. You could use COALESCE() to bypass the issue, converting the nulls to empty strings - or to any other constant value, for example '00:00:00' for the time part:

select concat( coalesce(cast(transactionDate as char), ''), 
               ' ', 
               coalesce(transactionTime, '00:00:00')
             ) as transaction  
from transactionTable 
where id = 123 ;


In fact, you don't really need an explicit cast(), the cast to char is implicit:

select concat( coalesce(transactionDate, ''), 
               ' ', 
               coalesce(transactionTime, '00:00:00')
             ) as transaction  
from transactionTable 
where id = 123 ;