minor #23803 [HttpKernel] Remove isset call used for legacy (ogizanagi)
minor #23803 [HttpKernel] Remove isset call used for legacy (ogizanagi)

This PR was merged into the 3.3 branch.

Discussion
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[HttpKernel] Remove isset call used for legacy

| Q             | A
| ------------- | ---
| Branch?       | 3.3 <!-- see comment below -->
| Bug fix?      | no
| New feature?  | no <!-- don't forget updating src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget updating UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | N/A <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

Added in #9628 in which the request stack dependency was nullable. Forgotten to be removed in #14634.

Commits
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e0a6010 [HttpKernel] Remove isset call used for legacy

symfony/HttpKernel,symfony/http-kernel