[Flight] Don't drop debug info if there's only a readable debug channel (#34304)

When the Flight Client is waiting for pending debug chunks, it drops the
debug info if there is no writable side of the debug channel defined.
However, it should instead check if there's no readable side defined.

Fixing this is not only important for browser clients that don't want or
need a return channel, but it's also crucial for server-side rendering,
because the Node and Edge clients only accept a readable side of the
debug channel. So they can't even define a noop writable side as a
workaround.