Set default JSON type resolver if not set

[captainsafia]

Closes #49360

[eerhardt]

FYI - If you wanted to write some tests for this, you could do them like this:

aspnetcore/src/Mvc/Mvc.Core/test/JsonOptionsTest.cs

Lines 29 to 44 in 238dabc

	[ConditionalFact]
	[RemoteExecutionSupported]
	public void DefaultSerializerOptions_SetsTypeInfoResolverToDefault_WhenJsonIsReflectionEnabledByDefaultTrue()
	{
	var options = new RemoteInvokeOptions();
	options.RuntimeConfigurationOptions.Add("System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault", true.ToString());
	using var remoteHandle = RemoteExecutor.Invoke(static () =>
	{
	// Arrange
	var options = new JsonOptions().JsonSerializerOptions;
	// Assert
	Assert.NotNull(options.TypeInfoResolver);
	Assert.IsType<DefaultJsonTypeInfoResolver>(options.TypeInfoResolver);
	}, options);

You would just be setting the bool to false.

[eiriktsarpalis]

Looks good. You can avoid creating a custom resolver if you just use JsonTypeInfoResolver.Combine() with no arguments.