This fixed a false-positive "ambiguous latency" warning for
MIDI busses:
MIDI track -midi-> MIDI Bus w/instrument -audio|midi-> master
The master-bus only has audio inputs, so the MIDI Bus'
MIDI out is left unconnected. Its latency does not matter,
it can float freely.
Previously it was assumed to be zero. So the MIDI Bus' input
playback latency range was [0, master-bus playback-latency].
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