MySQL Workbench Import NULL from CSV

If you were trying to import a CSV into the model for creating insert scripts (where you won't get the same options described in the other answers), try the following:

\func NULL

You can also use this syntax to call functions like the following to insert the current date time when you forward engineer the model to the database:

\func CURRENT_TIMESTAMP

Since MySQL Workbench version 8.0.16 (released on 04/25/2019) there has been an additional option for uploading .csv file -- "null and NULL word as SQL keyword". When selecting this option as YES, the NULL expression without quotes in .csv file (,NULL, rather than ,"NULL",) will work for this purpose.