Fix SortedSetIterable getFirst/getLast to return null on empty sets.

[motlin]

The sorted set implementations (TreeSortedSet, SortedSetAdapter, AbstractImmutableSortedSet) previously delegated getFirst() and getLast() to java.util.SortedSet's first() and last() methods, which throw NoSuchElementException when the set is empty. This violated the RichIterable/OrderedIterable contract which specifies that getFirst() and getLast() should return null for empty collections.

Changed to check isEmpty() before calling first()/last(), returning null for empty sets. Updated SortedSetIterableTestCase to remove the assertThrows tests that verified the incorrect exception behavior.