[HttpFoundation] Fixes a possible regression in SessionStorage top classes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (shouldn't be)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

This *should* fix a regression which was introduced by #9246 ; if this PR
was to set the session to "not started" was it was saved (and thus
closed), for an unknown reason, the PHPSESSID cookie was never
created for some apps (aka not the recent ones installed with the latest
versions of symfony).

But if we modify the `isStarted` method, and take into account the
`$this->closed` state, then it should enact the sought behaviour
without introducing this (possible) regression

I pointed this fix to master, but this regression (if this is one) is present since
2.2.10, 2.3.7, and 2.4.0-beta2.