threadnetwork: Avoid execute command if service does not exist

We might run VTS when the thread_network service is not available. This
patch prevents shell commands from being executed if the service is
absent.

Bug: 374009923
Bug: 355317617
Test: atest VtsHalThreadNetworkTargetTest
Change-Id: I94dd7e2e7585e061f53291c47e46841e5e670b6c