Set empty strings to null

[NoahHuntington]

How do I calculate field to replace empty strings with Null? I realize there is a Set Null tool available with the spatial analyst extension.  Without this is there another way?

[LeoDonahue]

Do a select by attributes on the field using '' as the expression to select empty strings.

Field calculate using: null  as the expression.

[NoahHuntington]

Generated "Not Nullable" error on field.  Where can I change this?

[LeoDonahue]

When you created your featureclass, and added that field, there was an option for allowing nulls.  It's probably set to No.  I don't think you can change it without recreating the featureclass.