Maybe it's a misunderstanding of "bar". A bar is simply an amount of time. Since the time is divided into bars, there is no time period with "no bar". And since a bar must normally cover at least one price quote, there is also no bar with "no price".

When a market opens at 6:30, then there's normally also a new price at 6:30, and you get a new bar at 6:30. When you set BR_FLAT, you'll get a new bar even without a new price.