Multiselect: field with empty value

I have a multiselect box on a filed-- modelName modelName can have different values or empty value. eg. `modelName="modelA" modelName="modelB" or modelName=""` modelName="*" I set the default multiselect token to `"*"` to select all of the records. But it will ignore `modelName=""`(the null case) Is there anyway to include the null case? Thanks.