Oversight in {rBb0741e1dcbc5}.
This was guarded by an assert in `get_target_element`, but it can be
valid to have these assigned to a shortcut (and then perform the action
without an active outliner element).
Now remove the assert and let the operator polls check if we really have
a target element.
note: this basically makes `get_target_element` obsolete, could call
`outliner_find_element_with_flag` instead in all cases.
Maniphest Tasks: T91411
Differential Revision: https://developer.blender.org/D12495
3c2e4f4cfd