Burndown charts still load events synchronously

In !10328 (merged), we made burndown charts load the event data asynchronously.

But as noted in #7302 (comment 233410799), we still load all events on page load because we call Burndown#empty? and Burndown#legacy_data? which needs all events to be queried and grouped.

For #legacy_data?, I think we need to change the call to #select and #any? here so that we don't need to load all the records in memory.

For #empty?, we need to also change burndown_events.any? into a DB lookup if possible. Or we could just check the results of the async request instead of doing this here.