modules/filetype: ensure that modules does not provide empty settings

[stasjok]

helpers.mkIfNonNull' checks only for value = null, but I noticed that when there is anywhere something like mkIf false config.filename = {} (like with vlang filetype currently in nixvim, see also #1907), value of config.filetype is not null, but has all attrs with null values. It can be debugged with nix repl:

nix-repl> c = makeNixvim {}

nix-repl> :p c.options.filetype.definitions
[ { extension = { _type = "if"; condition = false; content = { v = "vlang"; }; }; } ]

nix-repl> :p c.config.filetype
{ extension = null; filename = null; pattern = null; }

Notice that condition = false, but the value is not null.

This PR changes mkIf condition to account for that.

NOTE: Test checks.x86_64-linux.tests.entries.modules-filetypes will fail until #1907 is merged.

[MattSturgeon]

noticed that when there is anywhere something like mkIf false config.filename = {}, value of config.filetype is not null, but has all attrs with null values.

This is probably related to how the module system handles things; e.g. if filetype contains sub-options those options will always appear in the final merged config.

For our use-case where we often want to print entire attrs as lua this is annoying and is why toLuaObject filters out empty attrs by default.

[MattSturgeon]

Looks good, thanks!

[MattSturgeon]

BTW you can use lib.hasInfix when you don't actually need a regex:

Suggested change

	assertion = builtins.match ".*vim\.filetype\..*" config.content == null;
	assertion = lib.hasInfix "vim.filetype." config.content;

[stasjok]

Right. I already found out about it yesterday when working on modules/output refactoring. I intended to change it in other PRs, but looks like I forgot about this.