Bug: nil error on moving to next conflict when first opening the conflict file

### The problem 🐛
When I run `cn` my keymap for `GitConflictNextConflict` before pressing any key I get this error:

https://github.com/akinsho/git-conflict.nvim/assets/43537864/8df10b2a-e193-4e5f-a7b3-51257461b71d

Here I try running `GitConflictNextConflict` without pressing any key it and the error gets triggered, if I open it again and press `j` and the `GitConflictNextConflict` it works as expected
### A quick fix 🩹
Change this to cover `match.position` being nil
https://github.com/akinsho/git-conflict.nvim/blob/45cde6f0acf26d0e4b64acfa45349dd1da01e577/lua/git-conflict.lua#L343
```lua
if not match or match.positions == nil then return end
```
This does not handle the main problem that a key must be pressed for the `visited_buffers[0].position` to not be nil. I can make a PR for the quick fix, but I have to look more into why `visited_buffers` does not work when no key is pressed