BUG: Prevent crashes for TALib functions when stocks have nans.

If a stock stops gettign updated values, e.g. if a stock rolls out
of a universe strategy, currently the underlying batch transform
for TALib may have nans (which is another issue that could be addressed),
the nans cause crashes when passed to some TALib function, e.g. Bollinger
Bands are incompatible with all nan values.

So, drop sids that only have nan values for the current data panel.