Filling not applicable fields with N/A rather than leaving them empty or Null
Let me change the direction of the question...

If you are making an 'array' of Ec's as columns, that is poor schema design.  Instead, you should have another table something like this:

CREATE TABLE Ec (
    id ... -- id in the main table
    ec ... -- one of several Ec values
    PRIMARY KEY (id, ec)
) ENGINE=InnoDB;


There would be any number of rows in this table, depending on the number of Ec values you need.  There is no need for NULL or N/A.