client: Fix bluetooth_client_set_trusted() not working

The guard was guarding against a local variable ("device") being
NULL, instead guarding against one of the arguments ("device_path")
being NULL. Depending on the compiler and runtime, this would probably
fail most of the time, leading to problems using non-pairing devices
after the initial setup.

bluetooth_client_set_trusted: assertion 'device != NULL' failed

Fixes: 27e88fc0