Fix crash when key: omitted on transpose

[jymen]

When K: is omitted on score when transposing is requested an exception occurred preventing score display

X:1 T: sample A

When this is happening I think that "C" may be assumed instead as "none" to fix this problem

[paulrosen]

Thanks. I'll review this when I get a chance.

If you transpose a tune with K:none you should still not have a key signature after the transpose, right?

[paulrosen]

I'm sorry! I forgot about this pull request. I've since resolved the issue in a similar way to how you did it.

I hope to be a lot more timely in the future. Thanks for the contribution!