dupStringWithLen -> makeImmutableString
dupStringWithLen -> makeImmutableString

Refactor the `size == 0` logic into a new helper function that
replaces dupStringWithLen.
The name had to change, because unlike a `dup`-function, it does
not always allocate a new string.

rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix