musicxml2ly fails on line 824 -> dirty fix

Hello, I have a file for which `musicxml2ly` fails, because in line 824
variable `i2` is `None`.
Changing line 824 from `if i1 > i2:` to `if not i2 or i1 > i2:` fixed my
issue.

I'm sorry, but I don't have time to investigate this issue more (just went
in "don't think, fix it mode"). I can't guarantee that the file I worked on
is correctly formed. Nevertheless, I pass you the information. I can send
the file on request.

Thanks again for making Lilypond !
Élie