unified return null usages
[Doctrine Bridge] fix DbalSessionHandler for high concurrency, interface compliance, compatibility with all drivers (oci8, mysqli, pdo with mysql, sqlsrv, sqlite)
[HttpFoundation] update phpdoc of SessionHandlerInterface and unify parameters of all handlers according to interface
minor #10717 unified return null usages (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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unified return null usages

| Q             | A
| ------------- | ---
| License       | MIT

This PR unifies the way we return `null` from a function or method:

 * always use `return;` instead of `return null;` (the current code base uses both);
 * never use `return;` at the end of a function/method.

Commits
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d1d569b unified return null usages
bug #10720 [HttpFoundation] Fix DbalSessionHandler  (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] Fix DbalSessionHandler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

This is basically the same as #10652 for the DbalSessionHandler.

- First commit fixes fix DbalSessionHandler for high concurrency, interface compliance, compatibility with all drivers (oci8, mysqli, pdo with mysql, sqlsrv, sqlite).
- Second commit updates phpdoc of SessionHandlerInterface and unifies parameters of all handlers according to interface (so inheritdoc actually makes sense).

Commits
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524bf84 [HttpFoundation] update phpdoc of SessionHandlerInterface and unify parameters of all handlers according to interface
ccdfbe6 [Doctrine Bridge] fix DbalSessionHandler for high concurrency, interface compliance, compatibility with all drivers (oci8, mysqli, pdo with mysql, sqlsrv, sqlite)