Fix assert for Image Editor invert/resize operators

Caused by rB2bf4c74130ff.

For undo, the ImageUser's scene should be NULL (see D7022 for
discussion).

PaintTiles were already doing it beforehand in ED_image_paint_tile_push,
but Image ops [scale/invert] are calling
ED_image_undo_push_begin_with_image directly.

Now actually set the UndoImageHandle iuser.scene to NULL (rather than
asserting)

ref T75675

Maniphest Tasks: T75675

Differential Revision: https://developer.blender.org/D7435

source/blender/editors/space_image/image_undo.c

@@ -667,7 +667,7 @@ static UndoImageHandle *uhandle_add(ListBase *undo_handles, Image *image, ImageU
   UndoImageHandle *uh = MEM_callocN(sizeof(*uh), __func__);
   uh->image_ref.ptr = image;
   uh->iuser = *iuser;
-  BLI_assert(uh->iuser.scene == NULL);
+  uh->iuser.scene = NULL;
   uh->iuser.ok = 1;
   BLI_addtail(undo_handles, uh);
   return uh;