Enable empty lists inferred as double (eg: <double>[]) to return a zero of type double

[matthias-prevost]

I noticed that the CI is failing but doesn't seem to be caused by the modified code.

What was the problem ?

When using sumBy on an empty list with a type inferred as Iterable<double>, the method throws an error.
The error is :

type 'int' is not a subtype of type 'double' in type cast

Code in error :

N sumBy<N extends num>(N Function(T element) mapper) {
  ...
  return result as N; // located in: /lib/src/base/iterable_extension.dart:148
}

Step to reproduce :

<double>[].sumBy((e) => e)

Reason :
If the list is empty, the for loop will not be entered and the return will do return result as double; however result equals 0 and has type int at runtime.

Fix :
I propose to add an early return when the list is empty, we return either a zero from int or a zero from double depending on the type N is equal to. I had to create a small method that gets the expected zero since I did not find any better solution.