MySQL Index for searching for NULL values in text fields
Yes, a prefix length of one will allow you to have a working index for looking up NULLs.

Here's the SQLFiddle: http://sqlfiddle.com/#!2/12bf3/1/0

The EXPLAIN says that it is using the desired index.

Of course you might want to try a larger test to make sure it performs as well in reality as you hope.