average method
Returns the average of all the values in this iterable.
If value is provided, it will be used to compute the value to be
averaged.
Returns null if this is empty.
Example:
[2, 2, 4, 8].average(); // 4.
[2, 2, 4, 8].average((i) => i + 1); // 5.
[].average() // null.
Implementation
num? average([num Function(E)? value]) {
if (this.isEmpty) return null;
return this.sum(value) / this.length;
}