Do not assume left is non-null if another branch is null in splitLast (#24325)

Fix NPE in #24177

Author: hamzaremmal

library/src/scala/collection/immutable/RedBlackTree.scala

@@ -1201,8 +1201,8 @@ private[collection] object RedBlackTree {
       }
     }
 
-  private def splitLast[A, B](t: Tree[A, B]): (Tree[A, B], A, B) =
-    if(t.right eq null) (t.left.nn, t.key, t.value)
+  private def splitLast[A, B](t: Tree[A, B]): (Tree[A, B] | Null, A, B) =
+    if (t.right eq null) (t.left, t.key, t.value)
     else {
       val (tt, kk, vv) = splitLast(t.right.nn)
       (join(t.left, t.key, t.value, tt), kk, vv)

tests/run/sorted-sets-ok.scala

@@ -0,0 +1,19 @@
+import scala.collection.SortedSet
+import scala.collection.immutable.{SortedSet => ISortedSet}
+import scala.collection.mutable.{SortedSet => MSortedSet}
+
+@main def Test(): Unit =
+  val s1: SortedSet[Int] = SortedSet(1, 2, 3)
+  println(s1)
+  val s2 = s1.filter(_ % 2 == 1)
+  println(s2)
+
+  val s3: ISortedSet[Int] = ISortedSet(1, 2, 3)
+  println(s3)
+  val s4 = s3.filter(_ % 2 == 1)
+  println(s4)
+
+  val s5: MSortedSet[Int] = MSortedSet(4, 5, 6)
+  println(s5)
+  val s6 = s5.filter(_ % 2 == 0)
+  println(s6)