Lix shadow-internal-symbols deprecation causes build failure

[kittywitch]

https://docs.lix.systems/manual/lix/stable/contributing/deprecated-features.html?highlight=nix%20path-info#dp-feature-shadow-internal-symbols

       error: Shadowing symbol 'null' used in internal expressions is not allowed. Use --extra-deprecated-features shadow-internal-symbols to disable this error.
       at /nix/store/wnb63azsvs2pjq7c6dlqcrim4h9wi2ls-source/types.nix:180:48:
          179|     description = "derivation";
          180|     check = x: builtins.isAttrs x && x.type or null == "derivation";
             |                                                ^

Corresponding line: https://github.com/chessai/nix-std/blob/master/types.nix#L180

Actual cause, type "null" shadows internal null. https://github.com/chessai/nix-std/blob/master/types.nix#L244-L249

[arcnmx]

the line being flagged is actually an unintentional use of types.null from the rec, so swapping it out for the intended or _null is an easy fix at least (there's one more to fix in nonempty I think?)

the file does actually use the null type from the rec in nullOr though, that'll need to be changed to imports.types.null.

also who knows what happens if anyone dares to invoke with types;, I guess renaming the types.null may be something to consider...

[chessai]

willing to accept a PR for this.