Tezt/Cloud: Fix EOF handling

What

Fix EOF handling which is broken at the moment!

Why

It is broken!

How

The MR !14559 (merged) makes the handling better but it was still buggy. When pressing Ctrl+D, an exception is raised at top-level but not caught surprisingly. In any case, it was not caught by the eof loop.

The MR fixes this by making the next input returning a string option instead of a string. None will be returned once ctrl+d has been pressed once.

Manually testing the MR

dune exec tezt/tests/cloud/main.exe -- cloud dal -v --log-file /tmp/log --producers 1 --proxy

Once the proxy starts (after few minutes and logs are all blue), press Ctrl+D. Then rerun the command to re-attach. You can end the test by pressing Ctrl+C, wait a little bit, then Ctrl+D. Don't forget to destroy the machines.