Since 4.12 the use of typehints and return types has caused issues with
values fetched directly from config without validation. This has lead to
upgrade woes in a minor version (#10721) with no immediate recourse
other than manual system intervention.
To use types, we should ensure types, leaving a stable API that won't
error on a bad value - or should give a thoughtful and directive error
message if so.
Issue #10721 summary:
SessionMiddleware runs before FlushMiddleware
SessionMiddleware causes a PHP fatal error passing `null` to a `string`
parameter.
`null` comes from config, because default string value doesn't exist. We
need flush for this - but system execution never makes it that far.