bug: ArgumentNullException in OperationResponseProcessor

[oe-rene-kortmann]

Describe the bug

The returnTypeAttributes result can be null if MethodInfo is null which raise a ArgumentNullException in ToContextutalType.

NSwag/src/NSwag.Generation.AspNetCore/Processors/OperationResponseProcessor.cs

Lines 79 to 80 in 2a5ca6e

	var returnTypeAttributes = context.MethodInfo?.ReturnParameter?.GetCustomAttributes(false).OfType<Attribute>();
	var contextualReturnType = returnType.ToContextualType(returnTypeAttributes);

Version of NSwag toolchain, computer and .NET runtime used

Nswag: 14.2.0 but also tested with 14.6.3
.NET: 6.0

To Reproduce

Code example:

NswagArgumentNullProblem.zip

Expected behavior

Swagger is rendered correctly.