Stream with instrument but no notes crashes when writing MIDI

[jacobtylerwalls]

music21 version

dev

Steps to reproduce

p = stream.Part()
p.insert(instrument.Instrument())
p.write('midi')

Traceback

Traceback (most recent call last):
  File "/Users/jwalls/music21/music21/midi/translate.py", line 2533, in streamToMidiFile
    midiTracks = streamHierarchyToMidiTracks(s,
  File "/Users/jwalls/music21/music21/midi/translate.py", line 2420, in streamHierarchyToMidiTracks
    updatePacketStorageWithChannelInfo(packetStorage, channelByInstrument)
  File "/Users/jwalls/music21/music21/midi/translate.py", line 2357, in updatePacketStorageWithChannelInfo
    initCh = channelByInstrument[instObj.midiProgram]
KeyError: None

More information
Only a problem if there are no notes.
I think we assume parts with no notes are Conductor tracks. Simple fix.

Noticed responding to a m21 list thread May 2021.