Fix issue with NaN values in PolarPlots

[SamKry]

proposal fix for issue #409

respects NaN values and skips them

Open for discussion: What should happen with "connect first and last point" feature? Now it just connects the first and last non-NaN points (which could be misleading). Please just open a new issue for that and assign it to me if you would want to have this changed.

[trashgod]

I'm wary of silently ignoring NaN values by default. Would this feature see broader acceptance as a mutable property?

[SamKry]

I've updated the PR so that there is now a mutable property called hideNaNValues in DefaultPolarItemRenderer which controlls the handling of NaN values in Polar plots. (see discussion in issue #409 )

Additionally if connectFirstAndLastPoint is enabled, the new property is respected too. If hideNaNValues is enabled, it only connects the first and last point of they are not NaN, else a gap will be drawn.

hideNaNValues is set to false by default so that the default behavior is the same as before.

[trashgod]

For reference, this branch can be tested in a local clone, as shown here:

git clone https://github.com/jfree/jfreechart.git temp
pushd temp
git fetch origin pull/410/head:fix-NaN-values-in-PolarPlots
git branch --list
git checkout fix-NaN-values-in-PolarPlots
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