STL find ScrollBehaviorOwner if present

With the introduction of ag/29699552, node `SwipeToSceneRootNode` (and
consequentially `ScrollBehaviorOwnerNode`) is removed from the
composition if there are no scenes in that orientation.

Methods `nestedScrollToScene()` previously assumed the node was always
present and deferred to the STL to decide whether to consume the gesture.

Test: atest NestedScrollToSceneTest
Bug: 373802396
Flag: com.android.systemui.scene_container
Change-Id: I8dd09e5b9dfd834effd3ec8a228f5e0bf0a16f11