What are the requirements of your application. First of all, it looks liek the field is nto required. If that's not the case, then validate on the client to make sure something is entered before allowing form submission. If the field is not a required field, then is the column defined as not null in the database table?
Bosun (SCJP, SCWCD)
So much trouble in the world -- Bob Marley
Blank strings and null values are different for a reason, they have different logical meanings. Suppose you have a table with a field called "my_string_value" and allow your domain model to represent no value as a zero length string. How do you now query the database for all records where "my_string_value" is not set? You have to use database functions to do this, which can be inefficient and often database specific. If its null you can easily test.
Another thing to consider: in Oracle, you cannot store a zero length string. So if you have the same field you must default it to a single space (or some other suitable default).