Solve null date with case or if statement

I am wanting to process records for dates on an everyday basis. However I have a solution that will work incrementally but I need one that will load the first date if the table is null. Here is my query

  SELECT DateToProcess = MIN(MonthlyDate)
  FROM ods.CustomerBill b
  WHERE b.MonthlyDate > @DateToProcess

If @DateToProcess is null I want to set it to say 1999-01-01 that way it grabs the min date from my table and starts inserting records. How can I implement this additional logic to make this work?

Answer

Just wrap @DateToProcess with an ISNULL:

ISNULL(@DateToProcess,'19000101')