seat_get_focus() can return NULL when the focus stack is empty. a few
call sites don't check for this and just dereference the result, which
crashes sway.

this happens when a client disconnects abruptly, leaving the seat with
a stale has_focus flag or empty focus stack. whatever touches focus next
dereferences NULL. the two confirmed crash sites were
seat_set_workspace_focus and seat_set_focus_surface (coredumps below).
seat_unfocus_unless_client has the same pattern so a check was added
there too for safety.

cannot remember how this one was triggered:

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
wl_signal_emit_mutable ()
container_begin_destroy ()
view_unmap ()
handle_unmap ()
wl_signal_emit_mutable ()
wlr_surface_unmap ()
destroy_xdg_toplevel ()
destroy_xdg_surface_role_object ()
destroy_xdg_surface ()
xdg_client_handle_resource_destroy ()
wl_client_destroy ()
```

mouse click on empty container (clicked wallpaper, likely just after a
wallpaper engine crash):

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
handle_button ()
wl_signal_emit_mutable ()
handle_pointer_button ()
handle_libinput_readable ()
wl_event_loop_dispatch ()
```

IPC workspace switch (with noctalia workspace widget):

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
workspace_switch ()
cmd_workspace ()
execute_command ()
ipc_client_handle_command ()
ipc_client_handle_readable ()
wl_event_loop_dispatch ()
```

layer surface teardown (restarted noctalia shell):

```
seat_send_unfocus ()
seat_set_focus_surface ()
seat_set_focus_layer ()
handle_node_destroy ()
wl_signal_emit_mutable ()
sway_scene_node_destroy ()
layer_surface_destroy ()
surface_handle_role_resource_destroy ()
```

Set has_focus to false in seat_unfocus_unless_client when seat_get_focus() returns NULL.