Fix handling of top-level nullable type schema generation.

Author: eiriktsarpalis

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs

@@ -347,17 +347,30 @@ JsonSchema CompleteSchema(ref GenerationState state, JsonSchema schema)
             {
                 if (schema.Ref is null)
                 {
-                    // A schema is marked as nullable if either
-                    // 1. We have a schema for a property where either the getter or setter are marked as nullable.
-                    // 2. We have a schema for a reference type, unless we're explicitly treating null-oblivious types as non-nullable.
-                    bool isNullableSchema = propertyInfo != null
-                        ? propertyInfo.IsGetNullable || propertyInfo.IsSetNullable
-                        : typeInfo.CanBeNull && !parentPolymorphicTypeIsNonNullable && !state.ExporterOptions.TreatNullObliviousAsNonNullable;
-
-                    if (isNullableSchema)
+                    if (IsNullableSchema(state.ExporterOptions))
                     {
                         schema.MakeNullable();
                     }
+
+                    bool IsNullableSchema(JsonSchemaExporterOptions options)
+                    {
+                        // A schema is marked as nullable if either:
+                        // 1. We have a schema for a property where either the getter or setter are marked as nullable.
+                        // 2. We have a schema for a Nullable<T> type.
+                        // 3. We have a schema for a reference type, unless we're explicitly treating null-oblivious types as non-nullable.
+
+                        if (propertyInfo is not null)
+                        {
+                            return propertyInfo.IsGetNullable || propertyInfo.IsSetNullable;
+                        }
+
+                        if (typeInfo.IsNullable)
+                        {
+                            return true;
+                        }
+
+                        return !typeInfo.Type.IsValueType && !parentPolymorphicTypeIsNonNullable && !options.TreatNullObliviousAsNonNullable;
+                    }
                 }
 
                 if (state.ExporterOptions.TransformSchemaNode != null)

src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.TestTypes.cs

@@ -121,6 +121,18 @@ public static IEnumerable<ITestData> GetTestDataCore()
                 }
                 """);
 
+            yield return new TestData<int?>(
+                Value: 42,
+                AdditionalValues: [null],
+                ExpectedJsonSchema: """{"type":["integer","null"]}""",
+                Options: new() { TreatNullObliviousAsNonNullable = true });
+
+            yield return new TestData<DateTimeOffset?>(
+                Value: DateTimeOffset.MinValue,
+                AdditionalValues: [null],
+                ExpectedJsonSchema: """{"type":["string","null"],"format":"date-time"}""",
+                Options: new() { TreatNullObliviousAsNonNullable = true });
+
             // User-defined POCOs
             yield return new TestData<SimplePoco>(
                 Value: new() { String = "string", StringNullable = "string", Int = 42, Double = 3.14, Boolean = true },

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSchemaExporterTests.cs

@@ -41,6 +41,7 @@ public sealed partial class JsonSchemaExporterTests_SourceGen()
         [JsonSerializable(typeof(ReadOnlyMemory<byte>))]
         [JsonSerializable(typeof(DateTime))]
         [JsonSerializable(typeof(DateTimeOffset))]
+        [JsonSerializable(typeof(DateTimeOffset?))]
         [JsonSerializable(typeof(TimeSpan))]
 #if NET
         [JsonSerializable(typeof(DateOnly))]