Handle null byte when serving static files

[makushline]

This is raising an exception on a production application I am running. This should be fine since nil is an acceptable return type of the method.

[jkowens]

Rack has a utility method valid_path? that checks to see if it contains a null byte. If it's not a valid path that could be part of the return condition. Can you also add a test for this scenario?

https://github.com/rack/rack/blob/c8d0f2bb35d0abaab3a02fe7eec41bb2d9e68ddf/lib/rack/utils.rb#L598

[makushline]

@jkowens I missed your note and I had snoozed the errors on my application for some time. It's only when they came again that I remember about the open PR.

[jkowens]

LGTM, thanks! I'll see if @namusyaka can review 👍

[namusyaka]

I don't think the change is reasonable, left some comments.