Pose library: fix context menu

The Pose Library context menu was broken in the 3D Viewport's template
view by a958ae36e8cbe687c77f77b375a2dd21c01d271d.

That commit removed `asset_handle` from the context, and added
`active_file`, so now the pose library context menu code uses that instead.

pose_library/gui.py

@@ -62,7 +62,7 @@ def pose_library_list_item_context_menu(self: UIList, context: Context) -> None:
         list = getattr(context, "ui_list", None)
         if not list or list.bl_idname != "UI_UL_asset_view" or list.list_id != "pose_assets":
             return False
-        if not context.asset_handle:
+        if not context.active_file:
             return False
         return True