Convert HashMap<Key, OwnPtr<T>> to HashMap<Key, NonnullOwnPtr<T>>.

In every case I found, we never wanted to support null entry values.
With NonnullOwnPtr, we can encode that at the type level. :^)

Libraries/LibGUI/GTreeView.h

@@ -35,7 +35,7 @@ private:
 
     MetadataForIndex& ensure_metadata_for_index(const GModelIndex&) const;
 
-    mutable HashMap<void*, OwnPtr<MetadataForIndex>> m_view_metadata;
+    mutable HashMap<void*, NonnullOwnPtr<MetadataForIndex>> m_view_metadata;
 
     RefPtr<GraphicsBitmap> m_expand_bitmap;
     RefPtr<GraphicsBitmap> m_collapse_bitmap;