fix: unset eventignore with empty string instead of nil

[atusy]

Otherwise, the error like below occurs in recent neovim. I found this on nightly.

Error executing vim.schedule lua callback: .../.local/share/nvim/lazy/styler.nvim/lua/styler/theme.lua:82: Cannot unset global option value

:version

NVIM v0.10.0-dev-a935c75
Build type: RelWithDebInfo
LuaJIT 2.1.1699198486
Run ":verbose version" for more info

[folke]

Thanks!