comparison between nil and number when viewing diff with `sindrets/diffview.nvim`

[dlyongemallo]

Describe the bug

This bug only occurs when hlchunk.nvim is used with sindrets/diffview.nvim. In diffview, open a file with a diff, in particular with a deleted line on one side just before or after a scope (chunk). When moving the cursor past that line, an error occurs: Error executing vim.schedule lua callback: .../hlchunk.nvim/lua/hlchunk/mods/chunk/init.lua:168: attempt to compare nil with number.

To Reproduce
Steps to reproduce the behavior:

1. Edit a file in a git repo. Delete a line after a scope like {...}.
2. Open diffview.
3. Go to the side with the deleted line and cursor around the deleted line.
4. hlchunk display the above error.

Expected behavior
The cursor moves normally past the deleted line.

[dlyongemallo]

This will be fixed by #158.