Don't set context.Result to null on error detection

[benmccallum]

Request from @m.webert in Slack.

Potentially via a setting that defaults to current behaviour, as a minor release, then we can swap over in next major release the default.

[benmccallum]

Released in 6.1.0. Thanks to @markuswebert for the PR!

Unit test example of what this unlocks:

fairybread/src/FairyBread.Tests/InputValidationMiddlewareTests.cs

Lines 214 to 245 in a05871e

	[Theory]
	[InlineData(false)]
	[InlineData(true)]
	public async Task Should_Respect_SetNullResultOnValidationError_Option(bool setNullResultOnValidationError)
	{
	// Arrange
	var executor = await GetRequestExecutorAsync(
	options =>
	{
	options.SetNullResultOnValidationError = setNullResultOnValidationError;
	options.ShouldValidate = (ctx, arg) => ctx.Operation.Operation == OperationType.Query;
	},
	services =>
	{
	// Support those wanting to write data to context.Result in a custom
	// `IValidationErrorsHandler` implementation
	if (!setNullResultOnValidationError)
	{
	services.AddSingleton<IValidationErrorsHandler, CustomValidationErrorsHandler>();
	}
	});
	var query = @"query { read(foo: { someInteger: -1, someString: ""hello"" }) }";
	// Act
	var result = await executor.ExecuteAsync(query);
	// Assert
	var verifySettings = new VerifySettings();
	verifySettings.UseParameters(setNullResultOnValidationError);
	await Verifier.Verify(result, verifySettings);
	}