Fix null examples being represented as the string null (#3788)

Fix null examples being represented as the string null.

Co-authored-by: Javier García de la Noceda Argüelles <jgarcian@riamoneytransfer.com>

Author: jgarciadelanoceda

src/Swashbuckle.AspNetCore.SwaggerGen/XmlComments/XmlCommentsExampleHelper.cs

@@ -19,7 +19,7 @@ public static JsonNode Create(
 
         if (isStringType)
         {
-            return JsonValue.Create(exampleString);
+            return string.Equals(exampleString, "null") ? null : JsonValue.Create(exampleString);
         }
 
         // HACK If the value is a string, but we can't detect it as one, then

test/Swashbuckle.AspNetCore.SwaggerGen.Test/XmlComments/XmlCommentsExampleHelperTests.cs

@@ -80,8 +80,7 @@ public void Create_Returns_Null_When_Type_String_And_Value_Null_String_Literal()
         var example = XmlCommentsExampleHelper.Create(
             schemaRepository, schema, "null");
 
-        Assert.NotNull(example);
-        Assert.Equal("null", example.GetValue<string>());
+        Assert.Null(example);
     }
 
     [Fact]

test/Swashbuckle.AspNetCore.SwaggerGen.Test/XmlComments/XmlCommentsSchemaFilterTests.cs

@@ -76,8 +76,8 @@ public void Apply_SetsDescription_FromPropertySummaryTag(
         { typeof(XmlAnnotatedRecord), nameof(XmlAnnotatedRecord.ObjectProperty), JsonSchemaTypes.Object, "{\n  \"prop1\": 1,\n  \"prop2\": \"foobar\"\n}" },
         { typeof(XmlAnnotatedType), nameof(XmlAnnotatedType.StringPropertyWithUri), JsonSchemaTypes.String, "\"https://test.com/a?b=1\\u0026c=2\"" },
         { typeof(XmlAnnotatedRecord), nameof(XmlAnnotatedRecord.StringPropertyWithUri), JsonSchemaTypes.String, "\"https://test.com/a?b=1\\u0026c=2\"" },
-        { typeof(XmlAnnotatedType), nameof(XmlAnnotatedType.StringPropertyWithNullExample), JsonSchemaTypes.String, "\"null\"" },
-        { typeof(XmlAnnotatedRecord), nameof(XmlAnnotatedRecord.StringPropertyWithNullExample), JsonSchemaTypes.String, "\"null\"" },
+        { typeof(XmlAnnotatedType), nameof(XmlAnnotatedType.StringPropertyWithNullExample), JsonSchemaTypes.String, null },
+        { typeof(XmlAnnotatedRecord), nameof(XmlAnnotatedRecord.StringPropertyWithNullExample), JsonSchemaTypes.String, null },
     };
 
     [Theory]