Conditionally call jack_set_thread_init_callback_impl

[worikgh]

Do not call jack_set_thread_init_callback_impl from jack_set_thread_init_callback when thread_init_callback is None

See issue

[worikgh]

Whilst this solved the problem it set out to solve, the fundamental cause was the Jackd server not being build with the Berkeley DB.

Once the Jack server was built properly this cased to be a problem