pkgs-lib/formats: Add tests for serializing null values

In the effort to pass values directly as structured values, we ran into
issue with serializing null's.
This adds tests for null for all formats that support it and negative
tests for some others.

See https://github.com/NixOS/nixpkgs/pull/524404#issuecomment-4665694636

pkgs/pkgs-lib/formats.nix

@@ -1017,7 +1017,13 @@ optionalAttrs allowAliases aliases
     }:
     if format == "badgerfish" then
       {
-        type = serializableValueWith { typeName = "XML"; };
+        type =
+          attrsOf (serializableValueWith {
+            typeName = "XML";
+          })
+          // {
+            description = "XML value";
+          };
 
         generate =
           name: value:

pkgs/pkgs-lib/tests/formats.nix

@@ -142,6 +142,14 @@ runBuildTests {
     '';
   };
 
+  jsonNull = shouldPass {
+    format = formats.json { };
+    input = null;
+    expected = ''
+      null
+    '';
+  };
+
   yaml_1_1Atoms = shouldPass {
     format = formats.yaml_1_1 { };
     input = {
@@ -176,6 +184,15 @@ runBuildTests {
     '';
   };
 
+  yaml_1_1Null = shouldPass {
+    format = formats.yaml_1_1 { };
+    input = null;
+    expected = ''
+      null
+      ...
+    '';
+  };
+
   yaml_1_2Atoms = shouldPass {
     format = formats.yaml_1_2 { };
     input = {
@@ -210,6 +227,16 @@ runBuildTests {
     '';
   };
 
+  yaml_1_2Null = shouldPass {
+    format = formats.yaml_1_2 { };
+    input = null;
+    # nixfmt insists on removing indentation, so force it with ${"  "}
+    expected = ''
+
+      ${"  "}null
+    '';
+  };
+
   iniAtoms = shouldPass {
     format = formats.ini { };
     input = {
@@ -879,6 +906,14 @@ runBuildTests {
     '';
   };
 
+  cdnNull = shouldPass {
+    format = formats.cdn { };
+    input = null;
+    expected = ''
+      null: null
+    '';
+  };
+
   # This test is responsible for
   #   1. testing type coercions
   #   2. providing a more readable example test
@@ -989,6 +1024,14 @@ runBuildTests {
     '';
   };
 
+  luaNull = shouldPass {
+    format = formats.lua { };
+    input = null;
+    expected = ''
+      return nil
+    '';
+  };
+
   nixConfAtoms = shouldPass {
     format = formats.nixConf {
       package = pkgs.nix;
@@ -1014,6 +1057,15 @@ runBuildTests {
     '';
   };
 
+  nixConfNull = shouldFail {
+    format = formats.nixConf {
+      package = pkgs.nix;
+      version = pkgs.nix.version;
+      extraOptions = "ignore-try = false";
+    };
+    input = null;
+  };
+
   phpAtoms = shouldPass rec {
     format = formats.php { finalVariable = "config"; };
     input = {
@@ -1043,6 +1095,16 @@ runBuildTests {
     '';
   };
 
+  phpNull = shouldPass {
+    format = formats.php { finalVariable = "config"; };
+    input = null;
+    expected = ''
+      <?php
+      declare(strict_types=1);
+      $config = null;
+    '';
+  };
+
   pythonVars = shouldPass (
     let
       format = formats.pythonVars { };
@@ -1100,6 +1162,11 @@ runBuildTests {
     }
   );
 
+  pythonVarsNull = shouldFail {
+    format = formats.pythonVars { };
+    input = null;
+  };
+
   phpReturn = shouldPass {
     format = formats.php { };
     input = {
@@ -1145,6 +1212,11 @@ runBuildTests {
     '';
   };
 
+  xmlNull = shouldFail {
+    format = formats.xml { };
+    input = null;
+  };
+
   PlistGenerate = shouldPass {
     format = formats.plist { };
     input = {
@@ -1220,6 +1292,17 @@ runBuildTests {
       </plist>'';
   };
 
+  PlistNull = shouldPass {
+    format = formats.plist { };
+    input = null;
+    expected = ''
+      <?xml version="1.0" encoding="UTF-8"?>
+      <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+      <plist version="1.0">
+
+      </plist>'';
+  };
+
   hcl1Atoms = shouldPass {
     format = formats.hcl1 { };
     input = {