Fix all-NaN series stacking and Proxy return values.

Ensure that all-NaN series get treated as zero for stacking
purposes, and that nextPoint gets set to null past the last
stackable point. We want interpolation, but not extrapolation.

The Proxy class didn't return values from calls, breaking
testCorrectColors since it didn't get pixel data as expected.

Update the input data for testInterpolation to check corner cases,
including an all-NaN series.