eww: make configDir optional

[aster-void]

Description

eww's configDir does not need to be required, as some people (like me) prefer managing their dotfiles themselves.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.
  -> not yet

• Test cases updated/added. See example.
  -> marking as complete as I don't think this is necessary on this PR.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
    -> marking as complete as it doesn't add a new module.

Maintainer CC

@mainrs