fix HTTP 422 when gist-logs sends an empty file

I was getting an error when doing `brew gist-logs qscintilla2` and I eventually tracked it down to the fact that one of qscintilla2's log files was empty, and Github didn't like the empty string in the request. So I replaced it with the string "empty log". I'm not sure if that's the right solution, or to just leave that file out of the gist.