[13.x] Add initial value type to return of reduce and reduceWithKeys

[jnoordsij]

On empty collections, the reduce and reduceWithKeys methods will simply return the initial value:

> collect([])->reduce(static fn(): string => 'foo', 'bar');

= "bar"

Hence this should be part of the return typehint.

In real-world scenarios, this may appear when one does something like $collection->filter(<function where nothing passes>)->reduce(...) or $model->emptyRelation->reduce(...).

Note that the Enumerable interface already (correctly) prescribes this:

framework/src/Illuminate/Collections/Enumerable.php

Lines 882 to 892 in fa071b5

	/**
	* Reduce the collection to a single value.
	*
	* @template TReduceInitial
	* @template TReduceReturnType
	*
	* @param callable(TReduceInitial|TReduceReturnType, TValue, TKey): TReduceReturnType $callback
	* @param TReduceInitial $initial
	* @return TReduceInitial|TReduceReturnType
	*/
	public function reduce(callable $callback, $initial = null);