Runtime - initialise rand distribution of new threads

Previously, the thread local tracking the current thread's random number
distribution was only set to a specific value if the user explicitly called
use_/with_random_source.
(Otherwise, a white noise distribution was used _without setting the
corresponding thread local_).
With the introduction of a function to query the corrent random source,
returning a source of nil is undesirable - so here, when creating a new thread,
we query the thread local for the current random number distribution - _and
explicitly set it to :white if it is nil_.

Author: ethancrawford

app/server/ruby/lib/sonicpi/runtime.rb

@@ -1049,7 +1049,9 @@ def __in_thread(*opts, &block)
           __thread_locals_reset!(new_tls)
           __system_thread_locals_reset!(new_system_tls)
           __set_default_system_thread_locals!
-
+          unless SonicPi::Core::SPRand.get_random_number_distribution
+            SonicPi::Core::SPRand.set_random_number_distribution!(:white)
+          end
           __system_thread_locals.set_local(:sonic_pi_local_thread_group, :job_subthread)
           __system_thread_locals.set_local(:sonic_pi_spider_thread_id_path, new_thread_id_path)
           __system_thread_locals.set_local :sonic_pi_local_spider_users_thread_name, name if name