Japanese Character error + empty lists bugs

Hey @akiomik !
The last release was awesome. Much more user friendly the single executable file 👍

I tried to generate from many files to check if I could find more bugs.
I found 3, but 2 I don't quite know what's the problem.

### Non UTF-8 character
I tried to convert **purple_ext_original.dtx**, but it rose the following error:
`dtx2midi: Data.Conduit.Text.decode: Error decoding stream of UTF-8 bytes. Error encountered in stream at offset 65. Encountered at byte sequence "\145t\137\216"`
I noticed the problem was some japanese character at the file. The **003_purple_ext.dtx** is an adaptation that removed them and can be converted successfully :)

### Empty list: foldr1
While trying to convert **008_gensouya_ext.dtx**, I get:
`dtx2midi: Prelude.foldr1: empty list`
I only found this error on this DTX file.

### Empty list: last
While trying to convert **Smooth.dtx**, I get:
`dtx2midi: Prelude.last: empty list`
This particular error occurred on many other DTX files I tried.
Send me a message if you need more examples :)

You can find all the files above at:
[bug_dtxs_examples.zip](https://github.com/akiomik/dtx2midi/files/5147032/bug_dtxs_examples.zip)