`WM_gizmomap_group_find` can return null when removing ruler annotation
layer when current tool is not "Ruler" tool, this causes
`ruler_item_remove` to crash. This fix made it only remove ruler items
when `gzgroup` is not null (which is the same logic in
`view3d_ruler_remove_invoke`).
Pull Request: https://projects.blender.org/blender/blender/pulls/136416