fix: json null formatting in assertion report

src/Arcus.Testing.Assert/AssertJson.cs

@@ -216,7 +216,11 @@ private static JsonDifference CompareJsonNode(JsonNode expected, JsonNode actual
                 JsonArray actualArray => CompareJsonArray(expected, actualArray, options),
                 JsonObject actualObject => CompareJsonObject(expected, actualObject, options),
                 JsonValue actualValue => CompareJsonValue(expected, actualValue),
-                null => expected is null ? null : new JsonDifference(ActualIsNull, expected.GetPath()),
+                null => expected is null ? null : new JsonDifference(ActualIsNull, expected.GetPath())
+                {
+                    ExpectedNodeDiff = expected.ToString(),
+                    ActualNodeDiff = "null"
+                },
                 _ => null
             };
         }
@@ -470,8 +474,8 @@ internal class JsonDifference
         internal JsonDifference(JsonDifferenceKind kind, JsonNode expected, JsonNode actual)
             : this(kind, expected?.GetPath() ?? actual?.GetPath() ?? "<not-available>", expected: Describe(expected), actual: Describe(actual))
         {
-            ExpectedNodeDiff = expected?.ToString();
-            ActualNodeDiff = actual?.ToString();
+            ExpectedNodeDiff = expected?.ToString() ?? "null";
+            ActualNodeDiff = actual?.ToString() ?? "null";
         }
 
         internal JsonDifference(JsonDifferenceKind kind, string path, object actual, object expected)

src/Arcus.Testing.Tests.Unit/Assert_/AssertJsonTests.cs

@@ -539,6 +539,44 @@ [                    [
                 ""name"": ""post-metal""
               }"
                 };
+                yield return new object[]
+                {
+                    "{ \"this\": \"that\", \"options\": null }",
+                    "{ \"this\": \"that\", \"options\": { \"and\": \"this\" } }",
+@"Expected:    Actual:
+null         {
+               ""and"": ""this""
+             }"
+                };
+                yield return new object[]
+                {
+                    "{ \"this\": \"that\", \"options\": { \"and\": \"this\" } }",
+                    "{ \"this\": \"that\", \"options\": null }",
+@"Expected:          Actual:
+{                  null
+  ""and"": ""this""    
+}"
+                };
+                yield return new object[]
+                {
+                    "null",
+                    "[ \"this\", \"that\" ]",
+@"Expected:    Actual:
+null         [
+               ""this"",
+               ""that""
+             ]"
+                };
+                yield return new object[]
+                {
+                    "[ \"this\", \"that\" ]",
+                    "null",
+@"Expected:    Actual:
+[            null
+  ""this"",    
+  ""that""     
+] "
+                };
             }
         }