SignalBase: Don't terminate conversion when there's no data

SignalBase: Don't terminate conversion when there's no data

Instead of terminating, we wait instead.
We do this because SignalBase::on_samples_added() somehow doesn't
reliably see that there's no conversion thread active anymore.
conversion_thread_.joinable() returns true when the thread was
already terminated for whatever reason, resulting in on_samples_added()
trying to notify a non-existant thread.