nixos/mpd: correctly handle null valued cfg.settings

Tested with:

```diff
diff --git a/nixos/tests/mpd.nix b/nixos/tests/mpd.nix
index 36337bfbb4de..980dcca9048d 100644
--- a/nixos/tests/mpd.nix
+++ b/nixos/tests/mpd.nix
@@ -63,6 +63,7 @@ in
           mpd = defaultMpdCfg // {
             settings = {
               bind_to_address = "any";
+              db_file = null;
               audio_output = [
                 {
                   type = "alsa";
```

Per https://github.com/NixOS/nixpkgs/pull/485884#issuecomment-3865647552

nixos/modules/services/audio/mpd.nix

@@ -26,10 +26,9 @@ let
           "\"" + (toString v) + "\""
       )
     ) a;
-  nonBlockSettings = lib.filterAttrs (
-    n: v: !(builtins.isAttrs v || builtins.isList v || isNull v)
-  ) cfg.settings;
-  pureBlockSettings = removeAttrs cfg.settings (builtins.attrNames nonBlockSettings);
+  settings = lib.filterAttrs (n: v: !(isNull v)) cfg.settings;
+  nonBlockSettings = lib.filterAttrs (n: v: !(builtins.isAttrs v || builtins.isList v)) settings;
+  pureBlockSettings = removeAttrs settings (builtins.attrNames nonBlockSettings);
   blocks =
     pureBlockSettings
     // lib.optionalAttrs cfg.fluidsynth {