Bugfix for NaN values in bbroker data.close[0]

[1D0BE]

,This fixes the backtest broker displaying NaN for the portfolio value returned by get_value/getvalue, even if there are no positions in the data feeds where the data.close[0] value is NaN.

Practical Example:
One wants to backtest multiple data feeds of different assets over various times.
Backtrader is prefilled with all assets, but cannot invest in stocks that haven't IPOed yet, for example. Nevertheless, the data feed is already loaded.
This problem was also discussed here

Fix:

• add python standard math dependency for isnan()
• check for NaN when computing the portfolio value.
• continue the value calculation loop if the closing price of the asset is NaN, no matter the position. If there is one, its worthless anyways.

Potential loopholes of fix:

• None is not yet checked for
• Infinity is not yet checked for.

[1D0BE]

Actually, that was a bit too quick of a fix.
With this fix, NaN is not allowed within the line of values plotted at the end.
This creates then a defaulting to 0 when markets are closed. Leading to the plot to look like this:

Not desirable.
I have now fixed it locally by introducing the exception to the exception case, but there has to be a more elegant solution this monstrosity.

Any ideas?

if` math.isnan(data.close[0]) and not all([math.isnan(data.close[0]) for data in datas or self.positions]):
        # position inherently has no value.
        continue