just-standard-deviation and just-skewness require 2 numbers in the array

In https://anguscroll.com/just/just-standard-deviation, using an array with one number returns the error message "less than one value was passed to standard deviation" but it should probably say "two values" or simply return 0. Same with skewness.

Additionally, passing an array of equal numbers into skewness would return NaN, but should probably be 0.