Refactor filter descriptor type checks in SwaggerGen

[iskandersierra]

Pull Request

Given that the public API allows to register a filter in this form without any specific validation on not assigning the Type property:

genOptions.OperationFilterDescriptors.Add(new FilterDescriptor
{
    // Type = typeof(TdpHeaderParameter),
    FilterInstance = new TdpHeaderParameter(IRequestContext.UserIdKey, "The user ID."),
});

in which case, opening the swagger UI returns a NullReferenceException (in developer error page) with no reference to user source code.

Looking into the library source code, I found out that the "proper" way to do the registration is:

genOptions.AddOperationFilterInstance(
    new TdpHeaderParameter(IRequestContext.TenantIdKey, "The tenant ID."));

but I just felt that the first option should also be available with minimal behavior changes, given that it is a possible choice in the public API.

The issue or feature being addressed

NOTE: I didn't create a specific issue

Details on the issue fix or feature implementation

I added an alternative validation on the filter type for when the FilterInstance is set but not the Type of the FilterDescriptor.