fix(MultiMarketPower): guard OnCalculate forward-fill against bar 0

[AlbertoAmadorBelchistim]

Split out of #173 as requested in the review.

OnCalculate forward-fills empty values of the forming bar from the previous bar:

if (ds is ValueDataSeries vds && vds[bar] is 0)
    vds[bar] = vds[bar - 1];

When the forming bar is bar 0 (a chart with a single bar, e.g. right after the history response on a new instrument or a very short range), bar - 1 is -1 and the indexer is called with a negative index. The fix returns early for bar 0, where there is no previous value to carry forward:

if (!_bigTradesIsReceived || bar != CurrentBar - 1 || bar == 0)
    return;

No behavior change for any other bar. Single-line change, rebased on the current Develop.