Init NestedAnimation's nestedArtboard as nullptr

This should fix an issue with nested events not bubbling up on Windows and web editor.

Diffs=
c5fc07de9 Init NestedAnimation's nestedArtboard as nullptr (#7857)

Co-authored-by: Philip Chung <philterdesign@gmail.com>

.rive_head

diff --git a/.rive_head b/.rive_head
index b4f79f3..f62275e 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-fb7756072f7c28fff774e536b6bbbca664a35664
+c5fc07de9cc2d14a2d223397c8ea96d7393eb78e

include/rive/nested_animation.hpp

diff --git a/include/rive/nested_animation.hpp b/include/rive/nested_animation.hpp
index 70edc4d..ac14f36 100644
--- a/include/rive/nested_animation.hpp
+++ b/include/rive/nested_animation.hpp
@@ -47,7 +47,7 @@
     }
 
 private:
-    NestedArtboard* m_nestedArtboard;
+    NestedArtboard* m_nestedArtboard = nullptr;
     std::vector<NestedEventListener*> m_nestedEventListeners;
 };