Allow file middlewares to run if there's an endpoint with a null request delegate

[DamianEdwards]

This updates the DefaultFilesMiddleware, DirectoryBrowserMiddleware, and StaticFileMiddleware to not no-op (fall through to the next middleware in the pipeline) if they detect the request has an active endpoint with a null request delegate.

This allows the middleware to run in cases where there's an endpoint with a null RequestDelegate. This enables the ability to have an active endpoint for the purposes of communicating metadata to middleware while allowing middleware like the StaticFileMiddleware to keep performing its function, e.g. have a "dummy" endpoint to carry IAuthorizeData metadata so that the AuthorizationMiddleware performs authorization on a request before the StaticFileMiddleware runs (example here).

Fixes #42413

[davidfowl]

LGTM