PERF: Remove the drop of 'null return' from cumulative returns.

The check of existence of the null return key, and the drop of said
return on every single bar was adding unneeded CPU time when an
algorithm was run with minute emissions.

Instead, add the 0.0 return with an index of the trading day before
the start date.

The removal of the `null return` was mainly in place so that the
period calculation was not crashing on a non-date index value;
with the index as a date, the period return can also approximate
volatility (even though the that volatility has high noise-to-signal
strength because it uses only two values as an input.)
PERF: Reduce number of times the downside mask is created.

Assign the downside mask, `rets < mar` to a value instead of
calculating the downsides twice.