Motivation for this change

After 9786adf the nixos.tests.gitlab fails to instantiate:

[pbb@regolith:~/proj/nixpkgs]$ nix-build nixos/tests/gitlab.nix
error: A definition for option `services.redis.masterAuth' is not of type `string'. Definition values:
- In `/home/pbb/proj/nixpkgs/nixos/modules/services/databases/redis.nix': null
(use '--show-trace' to show detailed location information)

Since the masterAuth option has a default value of null, its type should probably allow for null as a value.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.