BUG: Fix change of type of period.sharpe

The factoring out of the Sharpe calculation changed behavior
so that both period and cumulative return nans when there is
no volatility; however before that change period returned 0.0.

This breaks existing consumers which expected a non-nan value
for period results.

Smooth out that change by checking the value after the sharpe
has been calculated and reset nan's to 0.0