fix empty line out of bounds in #361

[yingmanwumen]

Refactor function set_unmatched_lines in init.lua to make the logic more clear. Fix #361 as well. It has been tested in my local machine with neovim 0.9.0.

[gorillamoe]

Until the maintainer finds time to maintain the PRs in this repo again, I'll try to get maintenance / somehow critical stuff into this fork: https://github.com/mistweaverco/hop.nvim

I cherry picked your commit into the master branch of the fork already :) Thanks for your effort!