How to set a nullable field back to null?

[glenatron]

This might be somewhere in the documentation but I couldn't find it.

If I have a nullable field, say null.String and I have set it to a non-null value, how am I supposed to set the stored value back to null in the database?

[stephenafamo]

s := null.StringFrom("hello") // non-null string
s.Valid = false // set to null