Not much of an improvement, but I think

IF ((OLD.mast_Column_Name IS NULL AND
NEW.mast_Column_Name IS NOT NULL) OR
(OLD.mast_Column_Name IS NOT NULL AND
NEW.mast_Column_Name IS NULL)) THEN
EXECUTE PROCEDURE Audit_The_Change(parameters, ...);
ELSE IF (OLD.mast_Column_Name <> NEW.mast_Column_Name) THEN
EXECUTE PROCEDURE Audit_The_Change(parameters, ...);

would suffice (i.e. I don't see your first IF-statement making any
difference).

Set