man: make package nullable and default to null on darwin

tests/modules/programs/man/default.nix

@@ -1,4 +1,10 @@
+{ lib, pkgs, ... }:
+
 {
   man-apropos = ./apropos.nix;
   man-no-manpath = ./no-manpath.nix;
+  man-no-caches-without-package = ./no-caches-without-package.nix;
+}
+// lib.optionalAttrs pkgs.stdenv.hostPlatform.isDarwin {
+  man-no-package-on-darwin = ./no-package-on-darwin.nix;
 }

tests/modules/programs/man/no-caches-without-package.nix

@@ -0,0 +1,19 @@
+{
+  config = {
+    home.stateVersion = "26.05";
+
+    programs.man = {
+      enable = true;
+      package = null;
+      generateCaches = true;
+    };
+
+    test.asserts.warnings.expected = [
+      "programs.man.generateCaches has no effect when programs.man.package is null"
+    ];
+
+    nmt.script = ''
+      assertPathNotExists home-files/.manpath
+    '';
+  };
+}

tests/modules/programs/man/no-package-on-darwin.nix

@@ -0,0 +1,12 @@
+{
+  config = {
+    home.stateVersion = "26.05";
+
+    programs.man.enable = true;
+
+    nmt.script = ''
+      assertPathNotExists home-path/bin/man
+      assertPathNotExists home-files/.manpath
+    '';
+  };
+}