Fix 'no response returned' RuntimeError during cancellation in BaseHTTPMiddleware

[jonathanslenders]

Fixes: https://github.com/encode/starlette/issues/2516

If an incoming request is disconnected before a response gets produced, don't raise a RuntimeError. This fixes a race condition that occurs when many BaseHTTPMiddleware instances are wrapped within each other and the client (e.g., web browser) disconnects before the start of the response has been produced by the endpoint.

An alternative fix, possibly better, would be to always return a dummy Response and never raise the RuntimeError, regardless of whether _is_disconnected is set.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.