query filter on null/undefined values?

I'm trying to create a query with filters based around some dates but I'm not clear on how I can create a filter where I know a value might be null/undefined.

For my example, I have startDate and endDate in a data table.  Start is always defined.  End might be defined or might not be.  (If there is an end date, the record expires;  if not, it lasts forever.)

So generically, my query needs to be something like:
if startDate <= currentdate AND (endDate > currentDate OR endDate is null)

However, I can't figure out a way to accomplish the null/undefined check.  I've a tried a few things, but none of them remotely worked.  (Combinations of EQ, LIKE, null, undefined, "", etc.)

Any ideas?  It doesn't seem like this should be that hard...

Thanks,

Tony