MAINT: Makes RandomWalkSource emit midnight UTC events in daily mode.
BUG: Explicitly guard against empty bar data in history container
contstruction.

BarData can be falsey. in create_buffer_panel, the intention of the
check against bar_data was to see if it was passed at all, not if it was
truthey. In order to make that check more explicit, the check now
asserts that bar_data is not None.