[BUG] `plugins.lsp.*.filetypes` should be nil-able

[meck]

Field	Description
Plugin	All LSP Plugins
Nixpkgs	unstable
Home Manager	unstable

• [x] I have read the FAQ and my bug is not listed there.

Description

plugins.lsp.*.filestypes should be able to generate a nil lua value, currently any null value is filtered out, falling back to nvim-lspconfig default value which makes sense. However to activate a lsp for any filetype this should be forced to nil.

The scenario I'm looking at is enabling ltex-ls for all files (with autostart = false) and starting it manually using LspStart where needed. I don't know if an empty list should be transformed to nil or the rawLua type should just be enabled as possible type?

[MattSturgeon]

This is a bit more of a general issue in nixvim. Our to-lua generator currently assumes that null-or-empty value should not be rendered as lua.

There's a few historical reasons for this, but it leads to frustration in cases like this where we actually want to include a nil or a {} in the output...

[MattSturgeon]

One workaround is to use our "raw lua" feature:

{
  foo.__raw = "nil";
  bar.__raw = "{}";
}