Search for records where ExternalId = NULL

@None@"

I'm trying to find records that I have not exported. Logically they are the records where ExternalId = NULL, but I am unable to find a way to specify that in the search.  In looking for a solution I tripped over "Filtering Lists that Contain Null Values", which applies to select lists or multiselects and not what I am trying to do.

Here is some ineffective Java code that kind of shows what I need:

      SearchMultiSelectField stage = null;     stage = new SearchMultiSelectField();     stage.setOperator(SearchMultiSelectFieldOperator.anyOf);      stage.setSearchValue(new RecordRef[]{ new RecordRef(null, null, "@None@", null)});     customRecordSearchBasic.setExternalId(stage); 

Thanks,
Phillip