Matthews correlation coefficient should return 0 instead of NaN

[tridao]

🐛 Bug

By definition, "if any of the four sums in the denominator is zero, the denominator can be arbitrarily set to one; this results in a Matthews correlation coefficient of zero, which can be shown to be the correct limiting value."
However, the code does not check for this case and will return NaN.

[github-actions]

Hi! thanks for your contribution!, great first issue!