align a return type in collections internals with Scala 3

context: scala/scala3#18525, which upgrades Scala 3 to use the
2.13.12 stdlib; MiMa there is complaining

src/library/scala/collection/immutable/HashMap.scala

@@ -2181,7 +2181,7 @@ private final class MapNodeRemoveAllSetNodeIterator[K](rootSetNode: SetNode[K])
     curr
   }
 
-  override def next() = Iterator.empty.next()
+  override def next(): K = Iterator.empty.next()
 }
 
 /**