omit empty META_INSTRUMENT event when track.instrument is nil

Hi Jim,
I noticed midilib is generating unnecessary META_INSTRUMENT events. It was easy enough to avoid by checking if instrument is nil in seqwriter#write_instrument.

PS - sorry about the whitespace diff. Your tab settings seem odd from my perspective. I generally only use spaces and never tabs, to avoid unwanted whitespace diffs like this. BTW, did you know you can append ?w=1 to the URL in github's pull request screen, to ignore whitespace differences. Then you can see I simply added the "unless instrument.nil?" check.