collect: Return null from Spliterator.getComparator() for natural ordering

[nickita-khylkouski]

Summary

• Return null from Spliterator.getComparator() when comparator is natural ordering
• Check for both Ordering.natural() and Comparator.naturalOrder() (both singletons)
• Apply fix to ImmutableSortedSet.spliterator() and CollectSpliterators.indexed()
• Add unit tests for the new behavior

Motivation

Per the Spliterator.getComparator() contract, null should be returned to indicate natural ordering. The JDK stream implementation clears the SORTED flag when getComparator() returns non-null, which prevents the sorted() optimization.

This causes unnecessary sorting when calling stream().sorted() on naturally-ordered ImmutableSortedSet:

ImmutableSortedSet<Integer> sortedSet = ImmutableSortedSet.of(1, 2, 3, 4);
// sorted() should be a no-op, but wasn't because getComparator() returned Ordering.natural()
sortedSet.stream().sorted().collect(Collectors.toList());

Fixes #6187

Testing

Added 3 unit tests to ImmutableSortedSetTest:

• testSpliteratorComparator_naturalOrdering - verifies getComparator() returns null for natural ordering
• testSpliteratorComparator_customComparator - verifies custom comparator is returned
• testAsListSpliteratorComparator_naturalOrdering - verifies asList().spliterator() also returns null

Also verified locally that stream sorted() optimization now works correctly.