[HttpFoundation] Fixed NativeSessionStorage:regenerate when does not exists

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8460
| License       | MIT
| Doc PR        | -

**warning** I did not try to reproduce this bug, so I am not sure it fixes the issue. I just open a PR with the suggested patch. Tests for http foundation are OK. I am not sur it's the best way to fix this issue as I do not understand why symfony reach this code if $_SESSION is not set.