StatUtils.sum returns NaN for zerolength arrays

Key: MATH373
URL: https://issues.apache.org/jira/browse/MATH373
Project: Commons Math
Issue Type: Bug
Affects Versions: 2.1
Reporter: Roman Werpachowski
StatUtils.sum returns NaN for zerolength arrays, which is:
1. inconsistent with the mathematical notion of sum: in maths, sum_{i=0}^{N1} a_i will be 0 for N=0. In particular, the identity
sum_{i=0}^{k1} a_i + sum_{i=k}^{N1} = sum_{i=0}^{N1}
is broken for k = 0, since NaN + x = NaN, not x.
2. introduces hard to debug erros (returning a NaN is one of the worst forms of reporting an exceptional condition, as NaNs propagate silently and require manual tracing during the debugging)
3. enforces "special case" handling when the user expects that the summed array can have a zero length.
The correct behaviour is, in my opinion, to return 0.0, not NaN in the above case.

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