What function to use if I want three seperate results depending on true, false and blank

Hello,

I have two columns of data, if the two match I would like the result in the third column to be "QC OK', if they don't I would like the result to be "QC Fail", if the second column is left blank I would like the result in the third column to be left blank. The IF function only allows two results. What function would I use so I can get the third?

Thank you.