(cherry picked from commit 4b9e584)

The story is the same as in 471cffc:
device_attach() → seat_send_changed() → sd_bus_emit_properties_changed_strv()
→ emit_properties_changed_on_interface() → node_vtable_get_userdata()
→ seat_object_find(), which returns 0 because message == NULL.
But when we are emitting a signal, message is always NULL. Removing the
overeager check and assert in the called function allow the signal to be
emitted.

Fixes systemd#13769.

(cherry picked from commit 8cc64c2)

The same as parent commit, but users. This is the third and last
foo_object_find() function in logind, so I think that this particular
family of bugs is finally squashed.

(cherry picked from commit 8163b9f)

471cffc was committed on a debug VM where I
didn't have git set up properly.

(cherry picked from commit e48a1e3)